Mould in an enclosure is not an infection of the spider. It is a symptom of the environment: organic material plus moisture plus air that is not moving. Fix the third factor and the other two stop mattering nearly as much.
The three kinds you will see
- White fuzz on cork bark. Usually harmless surface mould on the bark's own sugars, common on new cork. It fades in a well-ventilated enclosure within a week or two.
- Mould on prey remains. Entirely avoidable. Remove boluses and uneaten insects.
- Grey or green patches spreading over the substrate. The enclosure is too wet for its air exchange. This is the one to act on.
The fix, in order
- Remove every piece of dead prey and any visibly mouldy substrate.
- Stop misting. Water down one corner instead, and only when the substrate has lightened.
- Check that nothing is pressed against a vent band, inside or outside.
- Move the enclosure off the floor and away from a cold outside wall.
- If the enclosure has ventilation on only one face, that is the actual problem.
Springtails, the quiet solution
A culture of tropical springtails in the substrate eats mould, mildew and leftover organic matter, and they are harmless to tarantulas. They need a damp base layer to survive, which you have anyway. It is the closest thing to a self-cleaning enclosure that exists.
Why cross ventilation prevents it
Mould spores are everywhere already, including in your living room. What decides whether they take hold is whether the surface stays wet. In an enclosure that takes air in low and lets it out high, surfaces dry between waterings even while the deeper substrate stays damp. That single difference is why a well-ventilated enclosure with a wet base gets less mould than a sealed one with a barely damp base.