Feather duster or acro polyp?

Just observing one of my new acro frags tonight (a smooth skin) and saw what looked just like a feather duster worm popping in and out of a bump on the frag. Every few seconds it would pop out with its fan and then quick as a flash go back inside. Although it was the only one of its kind on the frag, it blended in perfectly with the rest of the coral. Same color, just kind of an unusual bump. I'm not aware of any polyp that behaves like this but who knows? Is it just a very well camouflaged duster encrusted on the frag?

I would post a picture but I couldn't get a good one. I figure just a description of the behavior would be enough. Do acros ever have little feather duster like polyps that poke in and out of the coral?

 

It's a barnacle. Very common on maricultured (or frags of maricultured) and wild acros.

 

Interesting. Thanks for the help, Va. I put a little drop of super glue over it so it doesn't interfere with the frag.

 

There's no need to kill it but to each their own.

 

As a newbie, I make the assumption that anything I don't recognize is an immediate and deadly threat to everything in my tank.

But now I know! Hey, at least I haven't removed any of those freaky looking, but supposedly beneficial, bristle worms from my tank.