??? Was there supposed to be a pic attached? If so it didn't show.
If it's a leather or a porities it may be a shed starting.
Edit;
nevermind, the pic just popped up. Sarcophytons like that normally shed a skin periodically, when it gets ready the surface of the top takes on a shiny/waxy look. Usually the polyps retract while this is happening, but I bet that's what's happening. It's a little hard to tell from the pic, but if it looks like a thin film/skin is about to come off that's it. It will blow off in the current soon. If you have any sps or other delicate stuff down stream, it may be a good idea to suck the skin off gently with a turkey baster. Otherwise, if it comes off and lands on them, it can irritate some corals.