It is probably "yellow houseplant mushroom". The little balls are the start of a fruitbody, and mycorrhizal fungi do not produce fruitbodies very often from what I understand.
I have had the yellow mushrooms, bird's nest fungi, slime molds and others and they didn't cause any problems, they can be a sign of overwatering though. The best way to avoid fungus gnats is also to let the mix dry out more between waterings.