+1 on the Tropic Marin Pro Coral Cure!
I had the same thing happen to one of my acan colonies. It looked almost sucked in but not really. No tissue recession or anything like that. It was just not fully opened as usual. I tried moving it to another spot,blew on it with a turkey baster,lessened the flow,checked the rocks around it,took a power head and blew around the rocks it was on just to make sure there wasn't anything irritating it,moved it to a shadier spot and none of it helped. I eventually moved it back to the regular spot it was originally at,left it alone and just kept monitoring it. After a little over a week or so looking unhappy, I decided to dip it real quick(like a minute only lol) with TMPCC and the next day direct fed it even though it wasn't extending its feeders. Once it sensed the food it started eating and it literally came back to normal right after. I really don't know til this day why it did it or what was wrong with it but I noticed that feeding it helped a whole lot!
This acan colony is the only one that has shown this type of behavior out of all the other ones I have. Weird. It has done it twice already but has always come back to normal after a while.
Let us know how things are with your acan!