It's aptasia I'm telling you!
I had the same looking things growing like weeds in my tank on a piece of LR. I'd shoot boiling water at them and they would melt into a ball then grow back somewhere else the next day or two.
It doesnt look like a zoa at all. You can see the middle of it is tube like. Zoas dont have a tube going down, its more flat at the top where the mouth is.