Well from the picture it looks lika cyano outbreak. But pictures are hard to really tell. Is it hard and crusty or soft and spongy, does it lift up like a mat?
If it is hard and crusty and won't scrape off with your fingernail, it is a red coraline and a good thing. If it is spongy/slimey or comes up like a mat it is cyano. Since you said it's on the sand, I'm guessing cyano.
If it is cyano....Phosphates and low water flow are the main causes, too much overfeeding, bad/old bulbs are other reasons for it. The cure is to stop over feeding, increase the water current to the area and do water changes to remove the phosphates, running purigen or something like it will help as well. Remove as mush as you can with a turkey baster (Mark it FISH ONLY) replace the bulbs in your light if it is old and turn them off for a day or so.
If you don't address the cause it will come right back after you remove it. Even if you use Red slime remover or turn off the lights.
