Too bad your otherwise fine effort has come to this.

Caused by mold spores. Was this potting soil fresh from a new bag or was it from older soil stored in a damp place?

Too much watering and lack of air movement makes for more mold. That large of a setup may have caused the room humidity to go well above 50%.

Your seedlings need to be much closer to the light source.

With our cold, wet spring, not too late to start over from scratch with a disinfected setup.

Your set up needs more and closer light, cooler temps and a fan.

Most of your seedlings will collapse in just a few days

due to 'damping off'.