That is Dino’s. Which one not sure but as fast as they are move probably Small Cell Amphidinium. To be sure a must closer look under microscope will be needed. I battled them for 2.5 months and beat them with waterglass (silicates) dosing pods, and keeping NO3 at 20 and PO4 at .2 they are a pain and do not go into the water column like other Dino’s do.