I'm guessing low humidity, based on your first post.
My eyesight is too far gone to have seen something like that.
Good catch!
You could try a dome, but domes can/will cause damping off. In the past, I've used slurppy lids. The hole for the straw is usually big enough to allow some air circulation, (at least until the seedling out grows the lid. By then they are usually old enough to handle the low humidity).