I had a couple of worms matching this description precisely, and they
were leeches. They are difficult to catch in my tank because it has
slate attached to the back and they hide out behind the slate. I did
get one, photographed it, but don't have a web presence and can't post
pics here. This is probably one of the non-parasitic variety, as it
never has bothered any of my fish, shrimp or snails.

Hunt around for some photos of leeches on the net and you should be able
to identify it, if that is what it is. You may not see anything exactly
like it, but the body shape will be close enough to compare.

James