Those are Hydra, I personally don't know how to get rid of them, but type in Hydra in search box. There was someone else trying to get them out of their aquarium.
Hydra like Tlindsey said. They are harmless. When u vacuum ur tank just place it over the areas where they are at and will get sucked up. Or get a scrub and gently rub the area and vacuum. Make sure ur tank parameters are good.