A lot of good info in this thread if it is infact amphidinium dinos. I waited a long time to test for dinos thinking it was just some sort of brown algae. Before it really get's out of control it's probably best to get a cheap microscope to know for sure since dino treatment is not fun.
To combat I'm currently raising NO3 and PO4, added a ton of copepods / amphipods and siphoning my entire sand bed daily through a 10 micron sock, and slowly removing it at the same time. There are many other potential treatment options discussed in this thread, however.
https://www.reef2reef.com/threads/a...treatment-methods.365850/page-16#post-5169677