If it's cyano, treating it depends on the age of the tank. All new tanks go through it, and if you continue good husbandry & maintenance practices it eventually goes away as the tank matures. You can suck out all you can with airline tubing when you do water changes. If it's an older tank, you'll need to make sure you have 0 TDS water going in, you have good flow to the area it's in, your light cycle isn't too long, you don't have too many fish, your parameters are good (and stable), and that you're not overfeeding.
Does it come off easily? You should remove it from that coral by blowing it off with a wave of your hand or a gentle blast of water.