looks like dinos, best determined under a microscope. Do not play with nutrients which is one of the reason dinos are multiplying. When we see zero readings, automatically we assume this is the cause but by the time you see zero numbers, its because the dino has consumed the po4 and no3 and are multiplying and in turn many dose no3 and po4 to bring numbers up not realizing they are feeding these flagellates even more.
Its biological deficiencies that are causing the dino structure and tank is already affected. Its important though to identify the type of dino for most effective battle.
No light is first key followed by the addition of bacteria to overcome the bad bacteria allowing them to thrive
Prepare by starting by blowing this stuff loose with a turkey baster and siphon up loose particles. Turn lights off (at least white and run blue at 10% IF you have light dependant corals such as SPS) for 5 days and at night dose 1ml of 3% hydrogen peroxide per 10 gallons for all 5 nights which works as an oxidizer. If you dont have light dependent coral- turn all lights off. During the day dose 1ml of liquid bacteria (such as micro bacter 7 or XLM) per 10 gallons. Clean filters daily and DO NOT FEED AMINO OR ADD NOPOX which is food for dinos, however you can feed coral, food which will help no3 and po4 to increase. If increasing nutrients, try to keep no3 to about 5 until you are done battling these cells.
Doing a daily siphoning will help greatly But . . . . . Siphoning will reduce nutrients , so siphon the water into/through a filter sock and save the water and return it back to tank. Obviously clean the filter sock each time.
You can feed fish as normal and if doing blackout, ambient light in room will work for them
