Sure looks like dinoflagellates.
If that's what it is ... you have my deepest sympathies. It's extremely difficult to get rid of.
If you siphon it away, does it come back within a few hours? I found dinos would have an immense spread rate.
It's toxic to anything that eats it, so control is very difficult. It's basically a red tide ... I believe red tide is also caused by a dinoflagellate.
The zooxanthellae in your corals is also a dinoflagellate, so it's conceivable that conditions favourable to the scum is also good for the good stuff, which is
maybe why your corals seem to look good (note - I'm just speculating). But it also means there's no silver bullet ... any kind of med that targets dinos is going to do a bad number on corals and anemones as well.
Anyhow, I've had the misfortune of having a dino bloom in my tank twice now. Both times almost caused me to tear down the tank and quit. In the end, leaving my lights off for about a week (and I mean, no lights, no actinics, no halides, no ambient sunlight, etc.) seemed to knock it back.
However I think that didn't work for Christy, so .... I'm not sure how effective it is in all cases. And of course, having no light is not good for corals and anemones that rely on light ... so ... calculated risk.
Good luck.