Black substance covering frag plug..

[Mike in CT]

Hi everyone,
I have some zoa's that are on a frag plug that is mounted on a piece of rock, I have had it about a year. Over that year, the plug and surrounding area has had this blank substance cover it... Any ideas what it is? Here is a crappy picture.
Thanks!

 

[Oscaror]

Could you get a better picture? Might be slime algae.

 

[reefwars]

That is a sponge species commonly found around zoanids

 

[Mike in CT]

Thank you! After closer inspection I can see its a sponge!