brown feather duster type thing?

I found this little nut looking thing in my tank. It's been sitting in there not doing anything for a while, I think it came with the live sand. I was looking at the common IDs and some other stuff to see if I could figure out what it is.

Its brown, has kind of a stock to it and little feather type things on the end. Feather dusters are white right?





Should I take it out or leave it?

 

I think mission called it.. aiptasia's will live in shells and anything they can find.
Ok for a while, but will easily take your tank over.

 

Ah yes. I have heard of those things before. Idk why I didn't think to check what they look like. I noticed another one growing on my live rock, so I'll have to get rid of them.