calcium deposits maybe. I know I get baby feather dusters growing on the indside of the tank a lot too. Couold be eithe rof those. Look for a read tip some where and that will tell you if they are alive. - BTW make sure they are not pods either, these actually move.
Mine are hard to get off to, when they get larger they look like a snail shell but there are little hair things coming. Are these small feather dusters?
