Need to know what this is on my zoa

I am sure this is a normal part of the zoa but I have never seen this before on mine. My corals are all pretty new to me, only been the tank a month or so. I noticed this bubble looking thing in the middle of the zoa. Can someone tell me what it is or what it means when a zoa does this?



I know its a bit closed up in the pic, that's because I was having a problem with my Hydra and the light came on at 100%. Both of my zoas closed up when a few crabs bumped into them after this happened.