They're good. They're filter feeders, most kinds are harmless. Some varieties grow so quickly they can crowd out some zoas, but the kind you've got there doesn't seem to be that kind. Even the fast growing nuisance ones are easy to deal with. I have some pink stuff that is really pretty, but once in a while (every few months) I have to rip out some chunks of it so it doesn't suffocate my zoas.