Ist das schimmel?

if fuzzy, it's mold. probably not anything to worry about. avoid it with a better wet-dry cycle in your irrigation habits. it's stying too wet and it's growing shit. Random stuff growing on substrate isn't necessarily "beneficial" just because it doesn't cause any specific poblem. spritz some h2o2 on the surface... maybe a few applications if you don't want to see it. let it dry out more on top before you re-irrigate. this promotes better roots anyway. algea is possible too.. same thing.. not too big of a deal

Just some beneficial fungi. Leave it and let it do it's job.