Fungal growth is a normal part of the decomposition process. Any non-living plant material in a moist environment [or a dry one, for that matter] will eventually show fungal growth. Slime molds are a nuisance, as they rapidly spread across glass in their "animal" phase, but they rapidly die back once they reach their fungal fruiting phase. Mushrooms of all kinds will mostly grow invisibly before sprouting with some regularity, usually from wet wood. Other fungi will mainly appear in the form of molds on dead leaves, bark, or soil/mulch. Some of it can be controlled with ventilation and lighting, but for the most part I wouldn't be too concerned. It's the parasitic and infectious fungi which are problems, and THEY don't show up as molds or mushrooms.