菇 is a bound morpheme (b.f.) meaning 'mushroom'. You never see it alone in modern Chinese — it always appears as part of a compound like 蘑菇 (mógu, the generic word for mushroom), 香菇 (xiānggū, shiitake), or 金针菇 (jīnzhēngū, enoki). Use 蘑菇 when you just mean 'mushroom'; otherwise attach a prefix to specify the type. Never try to use 菇 by itself.