Its a bacterial mat = a combo of nutrients and excess light
Is this tank at or near a window? Also check your phosphates and nitrates for elevated levels.
To eradicate, you will need to scrub rock with a firm toothbrush or auto detail brush which is stiff and siphon up loose particulates. reduce white light intensity for at least a week and add 1.5 ml of liquid bacteria daily for a week. At night, add 1ml of peroxide per 10 gallons and clean filters daily.
As it sunsides, add a few Cerith and astrea snails plus about 10 blue leg hermits which are tiny.
A pencil urchin may also work but theyre sensitive to nitrates.