Is the stuff dusty on the sand or sticky like you can move like a mat. Dusty=diatoms, mat like= Cyano.
Mine was sticky and like a mat, and never had diatoms that bad on sand. Some do, I never did. Doing a few things for mine, including adding a fighting conch that is making an improvement, just taking a few days. My fish guy also just told me to dial down my white to 65% so let's see if that has any impact which I could see happening because area not in direct light are not as affected at all. Behind my rock should be icky and mine are squeaky clean. Time will tell.
Good luck