What is this growing between my zoanthids??

[Ser Davos]

Full size: http://imgur.com/Ap8EiTb.jpg

Some mentioned it could be chaeto but I'm not so sure... it is definitely growing/spreading some and I would like to know what it is before I plan my attack. So far it is limited to this piece of rock that the zoas came on.

 

[EMUReef]

What does it look like when they are closed up? Does kind of look like chaeto.