if it's something eating it you have to find the culprit and remove it. it might be a worm or bug/pod of some sort living close by or even inside the coral.
if it's slowly degrading/rotting then you need to excise the infected or rotting flesh down to the healthy flesh, i.e. gangrene treatment. you're better off removing some healthy flesh even after cutting all the infected flesh just to be sure.
even if it's the latter, eaten/bitten flesh may need to be cleaned and treated, just like a human wound needs dressing and antiseptic. i like to use iodine (e.g. lugol's dosing) as the antiseptic and some flow to keep the wound "aerated".
sarcos tend to have the most "fat reserves" of corals so they can survive prolonged illness or attacks like this so you have that going for you. but it usually needs to be addressed by the hobbyist rather than letting it run its course (either way, coralivore or illness).
post a pic and maybe we can determine which it is from the appearance. hth