It look like a kind of dinoflagellates... Take a turkey baster, and blast the rocks...
Little-matter isf it's diatoms or dinos, starve it, by reducing nitrates and phosphates near 0. Try to run GFO
, and GAC
if possible... Use RODI
water when you do water changes, make more water changes to lower nutrients, vodka dosing start to work well on my tank for fighting hard to get rid dinos.
Blasting (spray.... I don't know the word in english) rocks with a turkey baster once per day, you should see white crap coming out from rocks, these release PO4
... That is detritus, dead exoskeletons, limestone, nitrates, and fish poo. Make sure you run the protein skimmer while doing this. Clean the filter sock often too...
Give a shot of turkey baster straight before a water change, it should help too. If you have sand bed, try to vaccum it while doing a water change, you should have brown nasty water coming out from here.
I hope this helps.