Mould and Fungi
Mould and fungal growths can be caused by several different reasons.
Unless the enclosure gets entirely swamped none are particularly harmful. In many cases your isopods may even like eating them.
With the exception of flowerpot fungus, a thriving springtail population in your enclosure will significantly help in control moulds and fungi. If you don't have springtails (of whatever sort) we would strongly advise getting some. The standard tropical or temperate springtails are inexpensive and incredibly useful for the general health of your enclosure.
Flowerpot Fungus
What to look for
Clusters of yellow balls 1-2mm in diameter under the soil. It can sometimes also show up on the substrate or sphagnum surface.
Why and how they turn up
This fungus is commonly found in any soil or compost. It will most likely be present in very low amounts in any healthy substrate. Once you see the clusters of yellow balls, things have gotten to the point where treatment may be necessary.
Danger level
None. Although your springtails and isopods won't eat it, it also doesn't pose any harm.
Prevention
Good ventilation/moisture control and occasionally disturbing the substrate can minimise the risk, but unless you're using sterile substrate and never introduce anything non-sterile it will be a risk.
How to treat them
Remove all you can see, then stir up the substrate so there is airflow to where it was. It should then die back to a level where it's no longer visible.
"Old Man's Beard"
What to look for
This isn't an official name for this mould, but once you see it you'll know why we call it that. White fluffy mould probably originating from supplementary food.
This also isn't one specific species, but a catchall for these types of mould.
Why and how they turn up
Almost always from food left in the enclosure too long. It is also one of the more common moulds that show up in new enclosures before the bioactive cycle has settled properly.
Danger level
None. You can even wet it down to make it easier for your isopods and springtails to eat it.
Prevention
Don't leave food in long enough to grow things. For new enclosures don't worry, it'll pass.
How to treat them
Just remove it. However, be very careful there are no springtails or mancae (or even adult isopods) in it happily munching away.
You can also remove the source and wet down the mould itself for them to eat.
Yellow Moulds
What to look for
Yellow powdery moulds on supplementary food, particularly non-flake foods like fish and silkworm pupae.
Why and how they turn up
Food left in the enclosure too long.
Danger level
None. Although some can be unhealthy for you.
Prevention
Don't leave food in long enough to grow things.
How to treat them
Just remove the mouldy food. Although check for springtails and mancae when you do.
Mushrooms
What to look for
Yep - mushrooms of one shape or another.
Why and how they turn up
Mushrooms are the fruiting body of various fungi. If the fungus in the soil is undisturbed for long enough it may produce mushrooms.
Danger level
None, although in theory some could be unhealthy for you.
Prevention
Springtails, good ventilation, and occasionally disturbing the substrate.
How to treat them
Pick them out, or squash them so they can be munched on by your isopods and springtails.