What's My Acropora Doing?

[CoralsAddiction]

I've noticed this feather looking part of an acropora opening and closing just a few mins ago. Anyone know what it is? It looks different from the coral's polyps.
Video: https://youtu.be/w9V4r5-UjIk
Photo:

 

[twon8]

It's a barnacle embedded in the acro

 

[Golfman12]

Yeah barnacle I have one in a chalice

 

[CoralsAddiction]

It's a barnacle embedded in the acro

Yeah barnacle I have one in a chalice

Thank you!

 

[Armon_p]

Barnacle for sure