I'm having a similar problem. Got a small green toadstool leather frag a couple weeks ago. It opened within hours and has been doing great since, until yesterday when it didn't open. Again today, it didn't open at all the polyps are fully retracted and the surface is smooth. It's not deteriorating that I can tell. My tank is only a few weeks old so I wonder if water quality is an issue; however, all my other corals are looking great. I tested the water and my nitrate is 0.5 (NH4 and NO2- both zero), pH 8.2 and steady. The one thing that seems off is my alk. I started dosing B-Ionic two part for the past three days. My calcium has gone up from 275 to 320 mg/L in that time, and my alkalinity from 10.2 to 11.2 (a bit high as I understand it). Could the high alkalinity be causing the sarcophyton not to open?