Mana is the life energy of the soul that is bestowed by nature. Everything is born with a certain amount, with humans containing the most. When something dies, it's body naturally decays and supplies the ground with nutrients. It's mana dissipates back into the environment.

However, there are times when orgone doesn't dissolve the way it's supposed to. Instead, the mana condenses into a thick and dense cloud called miasma. This pollutes the environment, corrupting it in various ways. Where it is heaviest, things cannot grow or grows sickly and weak. Creatures become rabid and hostile, and causes mutations in all living and nonliving things.

Why would this be the case that nature's own energy would be poisonous to itself?

Too much of a good thing can be bad for you

There are many real-life examples of things¹ which are "life giving" or crucial for life - in the right amount - but exposure to higher concentrations or for too long a duration can be detrimental to life or downright lethal. Additionally, sometimes too much of something isn't dangerous on its own, but it prevents or reduces access to some other vital thing.

Some quick examples include:

• 
  Oxygen, (as O₂) mixed with nitrogen, CO₂ etc. at the ratio of ~21% is essential to all land creatures, and it is possible to breath even 100% oxygen for limited time without any detrimental side effect. However:
  
  
  
  
  • Exposure to concentrations higher than 21% can lead to oxygen poisoning, which "[...] can result in cell damage and death" as the Wikipedia article puts it.
  
  
  • As ozone (O₃ - same element, different molecule), even concentrations of 5 parts per million (0.0005%) are defined as "Immediately Dangerous to Life and Health Limit (IDLH)"^[citation].
  
  
  
  


• 
  Sunlight in moderate amounts is "healthy" - it allows plants to generate oxygen, is important in vitamin D synthesis in humans etc. Too much of it will wilt plants, cause sunburns (and increased risk of skin cancer) in humans and can raise environmental temperatures high enough to cause brush fires.


• 
  Water are crucial for life. Imbibe too much and you risk water poisoning. Additionally, land creatures drown in water since as too much of it prevents access to air.

¹: I'm intentionally use the vague term "things" so it can cover anything from substances to energies etc. including magical / spiritual phenomena.

So, why is too much life energy dangerous?

If this mystical "life energy" is what powers life and encourages growth (in moderate amounts and concentrations), too much of it is harmful to living things as it accelerates wild and rapid growth:

• 
  Mild effects can be things like cancer and other various tumors (cells and tissues growing too fast or uncontrollably), partially blocked "pipes"² etc. This isn't immediately painful or even noticeable - but can lead to illness or death in the long term.


• At moderate levels, things get painful: skin and internal lesions occur as tissues separate due to unsynchronized growth. Fully blocked "pipes" lead to necrosis, suffocation etc. Plant life collapse under their own weight or die from malnourishement as their most exposed elements - leaves, flowers and fruits - grow faster then their stems/trunks and roots can carry or provide for.


• Exposure to extremely high concentrations of life energy is horrifying, painful and fatal: tissues grow at rapid rates causing tears and fissures (think of bone growths developing in minutes or seconds, piercing through organs, muscles and skin). Organs function out-of-sync causing all sorts of mayhem to the creature (hormonal cascades, blood pressure so high it ruptures vessels and squirts through orifices, digestive fluids eating through the digestive tract etc.) - and weird, malformed and nonfunctional limbs and organs sprout all over the creature (and inside it...). Worse of all, at this level of life energy, a creature can survive longer than usual - prolonging the torture.

²: This covers digestive tracts, respiratory tracts and blood vessels in vertebrates, sap tracheids in plants, breathing trachea in insects etc.

Because fresh food is healthy but rotten not

Why eating meat or vegetables give us nutrients but when we eat them rotten we get sick due food poisoning? The same applies to mana/orgone, their rotten state damage the environment.

Because drinking water is healthy, but much drown you

You said that rotten mana condenses into a thick and dense cloud, that means the concentration of mana increased a lot. Drinking a glass of water is healthy, but how about drinking 10 gallons of water? Amount matter, too much water can drown you or water intoxication.

Mana could be like oxygen. Oxygen has free radicals and produces ROS, both things produce damages in our mitochondria, DNA and proteins. Normal doses of oxygen aren't a problem, but elevated ones can produce hyperoxia:

Associated with hyperoxia is an increased level of reactive oxygen species (ROS), which are chemically reactive molecules containing oxygen. These oxygen containing molecules can damage lipids, proteins, and nucleic acids, and react with surrounding biological tissues. The human body has naturally occurring antioxidants to combat reactive molecules, but the protective antioxidant defenses can become depleted by abundant reactive oxygen species, resulting in oxidation of the tissues and organs.

That increases the risk of cancer, which explains why higher doses of mana grow sickly and weak plants. Maybe mana is radioactive, like banana radiation, but higher ones are risky.

Because it pushes the local ecosystem out of balance.

Think about plant growth. In general, things that promote growth are good. We get thriving forests, plains, deserts, seascapes. We have an explosion of wildlife, clean air, fresh water, everything a planet could want (even if it gets in the way of certain humans).

But one of the worst things for the environment is ... plant growth.

In real life, plants get nitrogen from the air, minerals from the soil, water as needed, and it's all kept in perfect balance from microbial life, earthworms and other animals in the soil, pollinators, and more. New plant and animal material (especially manure) rots to provide fertilizer.

In our all too real but artificially imbalanced world, plant growth is seen as science and only about 3 primary elements: nitrogen, phosphorus, and potassium. Application of these can cause all sorts of problems on farms (and anywhere the runoff goes, anywhere the water table touches).

What many don't realize is this happens unintentionally as well, far away from farms or gardens. Excess phosphorus—mainly from laundry and cleaning products—runs off into streams and water ways, it goes down drains into lakes, rivers, and the ocean. And it makes plants grow.

Phosphorus is one of the primary nutrients (along with potassium and
nitrogen) for plant growth. When phosphates were used extensively in
laundry detergents, the waste water carried the phosphorus to rivers,
lakes, streams and ponds. This led to massive algae blooms--a
condition known as "eutrophication." The algae depleted the water of
oxygen, which resulted in the deaths of large numbers of fish and
other organisms. (ref)

In your world, naturally occurring orgone sometimes fails to breakdown properly and it forms miasma. You don't say why this happens. Is it just random? Is it happening more often because of overpopulation or more use of magic? Or is the failure to breakdown happening in larger percentages because of something else humans are doing?

Regardless of the underlying mechanisms, miasma is like phosphorus, it spurs overgrowth and imbalance. Or perhaps you can compare it to mania. Healthy human minds are balanced but someone with bipolar has too much depression and too much "up." You might think the "up" part is desirable (you have tons of energy, get lots of work down) but it's actually pretty horrible to live through (though somewhat additive for some people).

Healthy bodies, healthy minds, healthy ecosystems don't have those ups and downs. They have balance and carefully tuned systems. This means they can recover from things that life throws at them. But if you push something out of balance often enough, it will harm it. Humans can recover from injury, but some injuries are too grave, or too repetitive to come back from. They can recover from the death of loved ones, but people who survive genocide are never the same again. Ecosystems recover from fire, but not from the determined reshaping of land and water by farmers.

Miasma is something living things can deal with, in small doses. But when it's a larger dose, or a more prolonged smaller dose, it's detrimental.

Because that's the way it is?

Consider the following question: "If eating vegetables gives you energy, why does eating rotted vegetables give you food poisoning?"

The answer? That's just the way it is. You, a human, just can't eat rotted food without getting food poisoning. Something good and wholesome has decayed and become bad. Both vegetables and rotted vegetables are entirely natural: That's just the way of it.

So too with your mana. It's gotten stuck in a dead thing. It has, for want of a better term, rotted. Exactly as you say it's turned into miasma and therefore does something different to the creatures around it in the same way that rotted food does different things to fresh food. The exact mechanism of the 'rotting' is up to you, but the reason that miasma is poisonous where mana is not is basically "They are different things".

At that point it's up to you to flesh out. I can't explain your mana, so I can't explain your miasma either. Luckily though: You don't actually have to flesh it out! Unless it's an essential part of your story (in which case I hope that you have more of an idea of what it is) the fundamentals of what mana/miasma are really don't matter. People know what the two things are, they know how one turns into the other, that's all they really need to know.