It's part of what has become ubiquitous in peat-based potting mediums.....Leucocoprinus birnbaumii......the yellow potting soil mushroom. The yellow stuff is part of the feeding and decomposing portion of the whole fungus organism, the mycelium and hyphae.

This is not a dangerous fungus, it is simply decomposing your potting medium and that's all. At some point, pretty little yellow mushrooms will appear; we call that the fruiting body of the fungus.

I wouldn't want Leucocoprinus in my containers and would repot at some point. I don't want my potting medium to be decomposed any more than it already is.