encrusting gorgonia is very resilient, if your lighting is adequate, and your water is even close, they'll be open again in a few days. they often close up, and cover over with a waxy looking mucus coat when the water gets stirred up, or somethings a little out of wack. i imagine it's a simple defensive mechanism. eventually(1-4 days) the mucus sluffs off, and the polyps resume normal activities.