Look for a leather-eating nudibranch. Not an easy task as they usually blend right in with the specimen they are eating.
A brief FW dip will knock it off, followed by a medicated tip in saltwater, to disinfect the wound. You can cut off around the damaged tissue if you wish, but typically we find if the parasite is removed, and the wound disinfected, the coral will heal on its own.
Looks like a fairly large specimen... if it can be done without tearing up your tank, that's how I'd proceed. If you can't safely remove the specimen without hardship, you may need to watch for the nudibranch, manually remove it (there could be more than one, BTW), and you can use a tiny bit of medicated dip or iodine supplement on the end of a Q-tip to carefully swab the injured area to disinfect it. DO NOT pour dip into the tank under any circumstances, but a tiny bit of dip on a Q-tip can clean the area.
Jenn