My first thought would be photosynthetic, colonial tunicates (though I don't know which from the pics at this point).
I've only seen them in one or two tanks on here - they're super cool, and they're rare in the hobby, but they do tend to be invasive from the few cases I've seen. I'd personally keep them on an isolated rock so you can contain them easily. Also, you'll probably want to remove them from the zoas - I don't know if they'd smother the zoas in time or not, but I wouldn't want to take the risk.