I have frogspawn and branching hammer. Sometimes they just close up. If one is closing up, and the others are OK, I would not worry too much. If they all close up for more than 2 hours, worry! Check your pH, etc. Keep polyfilters available at all times. They cure a plethera of ills.
Thanks, I slowed down the current in that corner of the tank. I'll see what happens next. It looks like it's getting worse. Gonna go test the water.......
If only one head is close up did that one come into contact with another coral or anything else? We did some moving around last weekend and one head touched a frilly mushroom and they both drew up, mine is frogspawn that did that and it is just now starting to come back out. Just an idea. I f one of my heads of hammer coral is too close to rock it never come all the way out either.