Ceratium is a genus part of dinoflagellates, microscopic planktonic plants. Famous for them typical shape, they are found in freshwater and marine environments. They are mixotrophs. That means they indulge in both the pleasure of predation and light feeding. Their "arms" serve them to float but are rather awkard to swim. They are not toxic and that's rather pleasant for a dinoflagellate, as they are well-known for having a tendency to bloom and create red tides. If you want to know more about them, check out the links below!
Pizay et al. 2009 Protist