I've had the same thing for a year or so. They must have smuggled in on some plants. I researched them awhile back trying to figure out what they were and came to the conclusion they are a type of leech that likes to stick onto snails and possibly other invertebrates. Every time i see one i pluck them out, but still, i know they are still around in one of my tanks.
Of course though, i could be wrong about them being a leech
Of course though, i could be wrong about them being a leech