It is probably due to unsterilized soil. There were probably spores in the soil and due to moisture and heat they started to grow. As far as if it will hurt the plants, I really don't know cause I've never had this happen to me!
I would watch where I get my soil from from now on cause there can be some pretty nasty things that WILL kill your plants in unsterile soil. Hopefully someone else can help you figure out if your plants will be okay but I'm really curious as to whether or not this will hurt the plants, so I'll be watching this thread.
EDIT: I'm assuming you're growing indoors...
DeZze