They often close for almost no reason. A bump or jar will close some for a week then you can frag one up and it'll stay open. Lol. As long as the skin is green it's OK. It's probably big enough now to start losing heads that are shaded out. Maybe make some nice 3 or 4 head frags. Good luck. I hope it's OK. Ohhh. They hate nitrate spikes.Have a 34 gallon reefer 170 with various sps, lps, softies, etc. 2 photon clowns and a biota Mandarin. Everything has been good for awhile. Been dosing allforreef for about 2 months maybe. I have a 40+ head Duncan that mostly closed up yesterday, and today it's totally closed. Everything else is fine except I lost 1 head on a hammer for some reason. No new additions in maybe a year. Try to keep all at 9, cal is 430, mag around 1300. No clue on phosphate or nitrate, I'll have to order new kits. Salinity is 1.026 via the tropic marin glass hydrometer. I've had this Duncan for like 4 years from a 13 head. Would an iodine dip help?