Sorry evidently I'm not the best at spelling, or editing....can anyone tell me what this is, I just recently noticed it on my rock, I believe it moved off a zoa frag I picked up. Another rock was covering it, so it's been in the dark.
Just guessing here since you dont know if it opens up or not...
Maybe try a flashlight in the dark?
Good or bad depends on it’s size... If large enough, it can potentially catch fish. I don’t think they reproduce like aptasia do though. I use to have a small one (about an inch fully open) in my tank for quite a few years with no massive outbreaks. Just the one, with no fish altercations either.