[Diana]

Its Cyanobacteria, an ancient and very resilient organism. Usually grows in aquariums with high nutrients, excess lighting and low flow (although I have seen it in higher flow areas).

I suggest as previously stated, remove as much manually with a siphon tube and then treat with Chemi Clean (follow waterchange directions).

To help curb future outbreaks remove more nutrients and waste, feed less, add more corals, add more flow, or reduce photoperiod. (Or combo thereof). It looks like you have slightly coarse sand? If you can safely gravel vac it that might help (it growing on the sand indicates that is where your nutrients are trapped). Sometimes you just get a stubborn batch that refuses to go away.

HTH
-Diana