[Myka]

Those are definitely flatworms. They will move, but some species are much more active than others. I'm not sure which species you have - some are predatory mainly to corals, some are prone to plague-like population explosions, and some are harmless. First things first, I'd check very closely that there aren't any on your corals. I'f the corals are clear, I'd keep an eye on their population, and if everything seems ok I'd just ignore them. Of the harmless flatworms, their population will wane and flux, and under normal conditions they aren't usually a problem. If they are on your corals, that's an entirely different matter. If you're noticing their population increasing quickly, that's a different matter too. If they are quite tiny - like 2 mm or so, then they are probably one of the harmless types, though I've had one occasion where a tiny type was infecting a Brain Coral (nothing else). I gave it a couple dips a week apart, and didn't see them again.