Looks like moss. I get them all the time, and I use regular potting soil! I've never lost any seedlings due to moss BUT I usually take is as a sign to reduce the frequency of watering. But it looks like their first sets of leaves are coming along nicely. Try bottom watering only if the moss is stressing you out; that might reduce the moss, as they seem to grown on moist surfaces.

Ok, that makes sense. I will cut back on watering. I've never seen what "damping off" looks like and I immediately panicked, thinking this is what might be happening, lol. Thanks for the advice. :)

Damping Off looks like the plant has been cut down at soil level. Your medium is too wet. Been there.

It looks like your mix is primarily peat, and having been kept too wet has grown algae, which is to be expected. Unfortunately it makes a crust, (another reason for watering from the bottom,) I will with a small knife carefully break up the crust, not disturbing the seedling, to help get air into the soil. Al

Yea, for sure let the soil dry out pretty good (but not too much) before watering again. For the flats you are using, if they are in/under close lighting like they should be, then give them a full 24 hours before watering after you see the top of the soil dry.

I have only been germinating seeds for two years, but I have always gotten some moss growth. It hasn't seemed to do any real harm, but this year I thought I would top the containers with fine chicken grit which I have seen mentioned in the forum as a way to counteract it.

I am also assuming that moss indicates lack of light since out in the real world it sure doesnt grow in full sun. - does that sound likely?

The grit surface will help prevent a crust developing, and because the grit dries quickly the growth of algae will be slowed considerably. With seeds known to be slow to germinate I also use the grit. Al