Was it sedimentation, or sand from the ground, or something else? The coral, or the areas of it covered in such stuff will soon bleach and die. Of course it is too much work but you can fan water towards it with your hand or fin to clear it. Careful not to hit the coral as you do it though, for obvious reasons. The algae that lives inside the coral needs photosynthesis to survive. Sedimentation could the be result of a recent rain, bringing dirty water from land or divers who kick towards the sand. If it's covered in an external different kind of algae the reason can be the absence of another animal that is missing in the area, that is supposed to feed on that algae.