maybe it's a rotting fungus or bacterial patch? how deep does it go? a very deep hole signals a parasite or coralivore.
most ime (hobby-wise) eat from the inside/out. ones that eat outside first are very fast eaters. you wouldn't have much of a coral left after a day or so.
the ones that eat inside/out can be dug out. usually, you cut at the opening/hole and you can either dig it out (very gross) or siphon it out with a strong turkey baster/siphon (only somewhat gross). a sharp exacto knife or surgical scissors are very good here. extreme infections may require completely lopping off the head to root out the parasite/coralivore.
then flush the area/cavity and i usually dose certain iodine supplements, more as an antiseptic-type of application than anything else. best to do this all outside of the tank in a separate holding bin with the tank water (during a waterchange is optimal). that way water parameters are identical and the coral sometimes doesn't even register it's been transplanted. moving it always submerged limits the stress.
otoh, if it's just a topical infection or fungus, i'd lightly scrape it away while siphoning the fungus or bacteria out. much like cleaning an infected wound. you can then up the flow a little to try and limit the chance of "re-infection". the iodine supplements are also applicable here. (i tend to prefer lugol's and kent's Tech I)
it looks more like a simple topical "infection" but that's only a guesstimate from that picture. hth
