Zoas Shed?

Every other week or so about half of my candy apple zoas will "shed" kind of like a Leather does then the next week the other half will. After the week is up they will open back up just as bright as before but i really dont notice a siz diference nor have I noticed any other new little zoas. The frag only has about a dozen or so polyps. Any ideas? Is this really a "shed"?

Note: I have other zoas in the tank aswell and colony of yellow polyps but I have never seen any of them do it.

Thanks in advance.

 

They may be cleaning themselves off. It is probably a sign that they are not getting enough flow.