You set up a new enclosure, get the substrate just right, add your cork bark and leaf litter, and within a week there is white fuzzy stuff growing on everything. It looks alarming. Your first instinct is to tear the whole thing apart and start over. Do not do that.
White mould is normal
That white fluffy growth appearing on your cork bark, leaf litter, and wood is almost certainly saprophytic fungi breaking down organic matter. This is what fungi do. You have put dead plant material into a warm, humid box. Of course things are going to grow on it. In a forest, this process happens constantly. In your terrarium, you just notice it because the box is small and the mould is eye-level on your shelf.
White mould in a terrarium is not harmful to your invertebrates. Isopods and springtails will eat it. Millipedes may graze on it. It is part of the decomposition process that makes bioactive setups actually work. If you have springtails in the enclosure, they will usually get on top of it within a few days and the visible mould will recede.
New setups are the worst for this because everything is freshly introduced and the microbial community has not balanced out yet. Give it two to three weeks. In most cases the mould peaks and then settles down as the ecosystem in the enclosure establishes itself.
When white mould is a problem
If the white mould is not receding after a few weeks and is spreading to cover large areas of the substrate surface, that points to a ventilation issue. The mould itself is still not dangerous, but the conditions allowing it to go unchecked (stagnant air, excess moisture, no airflow) are bad for your animals.
Increase ventilation. Drill more holes, add mesh panels, or prop the lid open slightly. You want some air exchange happening. The mould thrives in still, saturated air. Moving air dries the surfaces slightly and limits fungal growth without necessarily tanking your humidity.
Also check whether you are misting too heavily. A light mist on one side of the enclosure every couple of days is enough for most species. Soaking the entire substrate until it is waterlogged creates the exact conditions mould loves.
Green and black mould: pay attention
Green mould (typically Trichoderma or Penicillium species) and black mould (Aspergillus or Stachybotrys) are different. They indicate persistently waterlogged, poorly ventilated conditions, and unlike white saprophytic mould, they can indicate that the substrate has gone anaerobic (oxygen-depleted) at depth.
If you see green or black mould, check the substrate moisture level. Push your finger into the substrate. If it is soggy and smells sour or sulphurous rather than earthy, the lower layers have gone anaerobic. This is genuinely bad for burrowing species like millipedes and beetle larvae, which spend time deep in the substrate.
The fix is to improve drainage and reduce watering. For a mildly affected setup, increasing ventilation and letting things dry out a bit may be enough. For a setup where the substrate smells bad at depth, you are looking at a partial or full substrate change. Remove the animals, replace the affected substrate, and reintroduce with better drainage (a thin layer of clay balls or gravel at the bottom can help prevent waterlogging).
Mould on food
Food moulds fast in a warm, humid terrarium. A slice of banana will be furry within 48 hours. This is why you should remove uneaten fruit and protein foods within a day or two. Mouldy food attracts grain mites, fruit flies, and fungus gnats, and it looks terrible.
Leaf litter and wood going mouldy is fine because that is part of the decomposition process your cleanup crew handles. A piece of banana going mouldy is a different situation because it is introducing sugars and moisture to one spot, creating a localised bloom of mould and potentially attracting pests.
Beetle jelly in a dish is less prone to mould than fresh fruit but should still be swapped out every few days. If you notice mould growing on jelly that has only been there a day, your enclosure is probably too wet.
Mould on the animals themselves
This is the one situation where you should actually worry. Fungal growth on a living invertebrate (appearing as fuzzy white or green patches on the body rather than on the substrate) indicates mycosis, a fungal infection. This usually happens to animals that are already weakened by stress, injury, or poor conditions. The fungus is a secondary problem. The primary problem is whatever compromised the animal in the first place.
There is not much you can do to treat mycosis in invertebrates. Veterinary care for invertebrate fungal infections is extremely limited. What you can do is improve the conditions in the enclosure (better ventilation, correct humidity, clean substrate) and hope the animal's own immune system handles it. Sometimes they pull through, sometimes they do not.
Isolate any animal showing signs of fungal infection. You do not want it spreading to healthy animals in the same enclosure, and the affected animal will do better in a clean, well-ventilated setup where you can monitor it closely.
Springtails are your best defence
If you are not already running springtails in your invertebrate enclosures, consider it. Tropical springtails (Sinella curviseta or similar) eat mould, decaying food, and fungal spores. A healthy springtail population keeps mould in check, processes waste, and generally acts as the janitorial staff of a bioactive setup.
You can buy springtail cultures cheaply from most invertebrate sellers. Add a few hundred to a new enclosure and let them establish before adding your main animals. They reproduce quickly in the right conditions and are completely harmless to isopods, beetles, millipedes, and spiders.
Between springtails and sensible ventilation, mould is a non-issue in most setups. The real problems (green and black moulds, mycosis on animals) are symptoms of conditions that need fixing regardless of the mould itself. Fix the conditions, and the mould sorts itself out.