oh those guys are usually from the plants when you buy and if you miss when checking over it you get them . i'd remove it as it remind me of a leach.. and if it is a leach then i wouldn't want it to latch onto my fish
funny how yours is white.. mine that i always find are brown and or yellow.
I've seen some images of planarian that look like this. I'm not entirely sure if it's one of those or if it is a leech. I'll do my best to find it, but it is a very small creature in a very planted tank.