I agree with all the previous posts. It could very well be dinoflagellates, and if so you would treat it like any other nuisance algae. From Wikipedia-
Food inclusions contain bacteria, bluegreen algae, small dinoflagellates, diatoms, ciliates and other dinoflagellates
I would wave my hand in the tank uprooting it from whatever it's on, vacuum out whatever you can, and do water changes. I wouldn't be too concerned about this, more than you should be concerned about something like bryopsis. Actually, I'd rather have this than hair algae because hair algae doesn't want to let you vacuum it off.
I have had something like this before and it took about 6 months before it disappeared, using the above method of removal.