I know exactly what you are talking about, I always thought they were just caused by spiders.I just looked it up for you and it is apparently a type of mold that you need to treat soon else it will kill your plants.Here is the quote from the site:
"You have damp off--a type of mold-- and it must be treated immediately. It will kill seeds and seedlings. Sprinkle the entire surface of the soil with cinnamon. You must have made the soil too wet. Remove the humidity lids and get a
fan
on them so you dry out the soil a bit. Leave them alone for a few days or a week. Then you can spritz the seeds that are not up a couple of times a day Use a spray bottle and just enough of a spray to get the seeds damp."
I believe that applies to all plants and not just seedlings. I have seen it many times in my few years of gardening.