They will often close up for periods of time and shed kinda like a skin renewal process, Most times nothing to worry about, Once i had one stay like that for weeks on end. The algae growth on it does concern me though, You may try a little more flow towards it and see if it will shed that and reopen again.
Bill
I agree... Mine does this on occasion. Just long enough for me to worry about it and then the next day it's huge and gorgeous again. I agree give it a little more flow... Try to get the algae off gently... And wait.
Yea my toadstool does that all the time and always has come back after a few weeks. Atm mine has split itself.down the middle and looks majorly torn up. But when lights come on it fluffs back out with polyps extended and looks fine. They are a wierd coral