Looks like perfectly normal growth to me. As new leaves emerge as you have there it is normal for the lower one to be pushed/lay over. I usually snip some of the length of it off just to lessen the weight pulling against the stem.

You can read the FAQ here on Damp-off for more details but it attacks the stem at the soil line.

Over on the Growing from Seed forum there is also a FAQ on the white "mold" (actually a fungus you are seeing and what to do about it.

Dave

Onion seedlings let go of that first seedling leaf at about the time the second and third true leaves appear. When the seedling leaf starts looking funky I snip it off and remove it. Like Dave, once or twice a week I trim tops and withering leaves, which increases light penetration and keeps the seedlings upright while limiting possible diseases from rotting tissues. I also sprinkle a little dry seed-starting mix around the base of the plants once they are up and growing.

This all gets easier in a few weeks, when you can expose your onion seedlings to natural light in a cold frame. That's when they stiffen up and stop looking so wimpy.