The 3 lobes at the back end of the animal makes me think red planaria at that size. They can be pretty invasive, but they aren't NECESSARILY bad--just ugly, quick to reproduce, and can smother corals at higher quantities
Likely a type of Acoel flatworm. They can become a nuisance in a tank but not to the dance extent as the nasty red ones. I believe they are photosynthetic.