What is growing out of my gsp???

[Pntbll687]

So my gsp is growing like crazy. It has almost completely covered top and bottom of rocks. I noticed these "arms" growing out of the gsp on the underside of a shelf. Any help with what this is, will be much appreciated

 

[Crabs McJones]

So my gsp is growing like crazy. It has almost completely covered top and bottom of rocks. I noticed these "arms" growing out of the gsp on the underside of a shelf. Any help with what this is, will be much appreciated

My green star polyps did this as well, I believe it's the mat growing out to potentially attach to another rock