These are dinoflagellates. the battle is straight forward:
Prepare by starting with a water change and blow this stuff loose with a turkey baster and siphon up loose particles.
Turn lights off (at least white and run blue at 10-15% IF you have light dependant corals) for 5 days and at night dose 1ml of 3% hydrogen peroxide per 10 gallons for all 5 nights. If you dont have light dependent coral- turn all lights off.
During the day dose 1ml of liquid bacteria (such as bacter 7 or XLM) per 10 gallons.
Clean filters daily and DO NOT FEED CORAL FOODS OR ADD NOPOX as it is food for dinos.
Day 5,, you can start with blue lights - ramping up and work your white lights up slowly