Some other notes in regards to dinoflagates:
-Grows super quick. When manually removing it (syphoning, fanning off, etc.) it grows back within hours.
-Most herbivores won't touch it. Many dinos are toxic when consumed.
-It could appear as part of the post start-up cycle, as could diatoms and cyanobacteria.
-Slightly less resilient (easier to rid) then cyanobacteria.
If it's isolated to one rock, quarantine it with carbon in a seperate container.
Generally speaking, if it's a widespread problem, work on a solution to lower excessive organics and silicates. Cease any trace element dosing and a weekly 1-2 day(s) lights out helps. When growth is noticably subsiding, syphon out the remaining slime. This stuff may take up to a month or more to disappear in some cases.
Supplimental "bacteria tuning" can also help immensely in many cases. Prodibio, ZeoVit ZeoBac and extremely careful carbon dosing are some ways to handle this, if you have the means.
Good luck Alex, let us know how things work out!