Is my fish a nipper

Recently I have had concerns with the overall health of my two open brain corals. For the first couple of months they thrived but over the last couple they seem receeded (not poofy) in the daytime. However I have noticed recently (on my days off) that they look better first thing in the morning. I have also witnessed my Whitecheek tang nipping one, but maybe this was a one time thing? If they have been deflated for more than a few weeks wouldnt you think they have sufficient food and light and that this fish or maybe a flow issue was to blame. If its my fish is there anyway to make it stop?

 

if it is the fish there is no real way of stopping them

 

Thats kinda what I figured I just dont want to get rid of him and prob couldnt catch him anyways. Will the coral survive?

 

you can make a 5 sided box out of egg crate and put it on top of the coral

 

Thats a good idea, it would look bad though, I need a frag tank to put it in