Could be diatoms? Hard to say without a pic or any info about your tank. I’d try posting in the nuisance algae section. Include a pic of the problem, the age of the tank, what you use for flow and your water parameters.
it’s a real trick keeping a clean sand bed. Some folks use cucumbers or sand stirring gobies or nassarius snails to move things around. Some people stir it manually. My guess is that it may have more to do with flow or water quality.