The only way to be completely sure is to check-in under a microscope. That will also tell you what strain of dinos it is, which is crucial to eradicate them. For example example, ostreopsis dinos can be killed by UV. Amphidinium dinos won't go in the water column and are not killed by UV, but by inducing a diatom bloom to out compete them.
First thing would be identifying them (post your picture here or on the Facebook group "Dinoflagellates support group" for help) , but also checking your nitrates and phosphates and keep them away from 0 and stable. Dinos are part of your water chemistry, they will always be there. You'll have to "repair" that chemistry by out competing them. Adding phyto, live bacterias etc. when treating ibg/removing them is essential so something else can than dinos will replace that spot they were taking.