I don’t want to give you wrong info.
But the white fuzzies shouldn’t be a problem. I grow organic and I get the fuzzies every time I topdress with good worm castings (currently using Colorado Worm Castings from build a soil) and Craft Blend also from build s soil. If it’s not effecting the seedling you should be fine. It’s just the fungus breaking down the nutrients.
If it does hurt the health of the seedling. I would think it’s from over watering. *something I’m guilty of a well
If I’m wrong someone explain.