Over wet casing brings cinnamon mould, which DMR lists against casing that is too wet or too strongly disinfected. Over wet compost is worse: above 72 per cent moisture at spawning the compost goes airless and plaster moulds appear.
The opposite fault shows up as weepers. NHB describes leaking, stinking mushrooms where the compost is below 64 per cent moisture and the casing above it is wet.