ID critter in my Duncan

[SliderXC]

My son noticed this little guy popping in and out of the side of my Duncan coral. What is it and will it cause problems?

 

[Crabs McJones]

Barnacle, really cool filter feeder I dont think itll bother your duncan at all, it seems far enough down the stem

 

[SliderXC]

Thanks! I had never seen it before, but he must have gotten excited when I fed some reef roids.