They can shed for anything up to a week IME. They need loads of alternating indirect flow, enough to move the polyps vigorously but not bend them over fully so they can't remain upright. Water volume as opposed to velocity is what's needed. It seems very close to that return, it may be getting enough "flow" figures wise but in terms of the type of flow it could be totally inappropriate. As i said, laminar high velocity flow is as bad as none - these corals won't open in either of these scenarios. Direct a powerhead/pump right at a wall beside it, so the flow has to rebound off the aquarium wall to reach it. This will make the flow direction more chaotic and is appreciated by Sarcophytons IME. It appears the coral has been fouled by algae of some sort, which - if it opened before in that exact location - is probably the problem as opposed to a flow issue, it looks like cyano so perhaps try to brush it off gently with a soft toothbrush. If all else fails, try a FW dip of the same temperature and pH.
I have found in my experience that Sarcophytons are more difficult and finicky to keep than acropora! Yes, they are hardier, but their flow demands are even more specialised and their "shedding" mechanism is more complex and specific than stonies. They are more sensitive to pollutants and if the flow cant rid it of its mucous, then it will remain closed and refuse to open. Perhaps my experience is unique, though i think it probably isn't.
