Fixed your photo, you need to double click on the thumbnail (smaller photo) to get to the larger one, then post that one.
Anyway, it looks like you're experiencing a bit of tissue recession there. Personally I'd just break off the affected area, especially if it is the part that contains the worm. The worm, by the way, is probably a vermetid worm, which casts a mucous web to catch its food. This can sometimes irritate corals enough for them to recede.
Maybe someone can chime in here to tell you if you can save that piece that is closest to the main colony that is receding but the lower two bits that are mostly white already are probably a loss
