state the size of your tank, what kind of fish is in it, how long has the tank been running, water quality measurements, most importantly....... did you over feed your fish?
Without all the tank info, (and probably even with it) I would say they are planaria. They are not dangerous to your fish (and do make good food for them) but I think they do speak to a water-quality issue somewhere. I saw a post somewhere about this, but I can't recall right now. I'll see if I can find where I saw it.