Corals that sit low in the aquarium are often covered with sand. They may not get buried in the sand, unless you have a sand sifting goby or something similar, but more often than not, these corals do get a few grains on them here and there. While the grains themselves are harmless and won’t kill the coral, if they aren’t removed by water currents or physically removing them by the aquarist, they can block out light to the underlying tissues and cause slight to severe bleaching. The solution is simple, just keep your corals sand free, but it’s often hard to do when you have a goby that acts like its on speed half the time. If constantly dusting off the corals is no longer something you want to do, either remove the fish or relocate the coral to a higher spot in the tank.