There are more than likely planeria, free swimming parasites that get into your tank on plants, rocks gravel etc. As a rule of thumb, if they aren't on your fish, they shouldn't cause any harm. They sound like they could be little parasites called flatworms. Obviously, without seeing them that's a rough guess. Although they are unsightly, I shouldn't worry too much about them. Getting rid of them normally involve stripping the whole tank down. However, you could start harvesting them to keep the numbers down. Eradicating them totally by doing this is unlikely though. I've never had these myself, other people may have other ideas of how to get rid of them. I've had snails before, they are a real pain in the backside.
By the way, congratulations on being our 500th member
