Totally normal surface mold / fungus. It’s generally harmless, and it’s more notable not to have any in my experience. If it bothers me, I spray it down with 3% hydrogen peroxide… that takes care of it for a couple days, but it comes back. The seedlings don’t mind one way or the other.
The sure way to get rid of it is to use a mulch which keeps the wet down but stays dry on top. In my opinion, the seedlings are too delicate for mulch until hardening off (at least, my usual mulch of pea gravel). So I put up with the mold.