ID this please

[bornagain]

This rock was uncured and came with many corals sponges Clams/barnacles and other critters. Some die-off has occurred.
My tank has been cycling for almost 2 weeks. Nitrite has just started to appear yesterday. Wondering if this guy will make it. He just started pulsating today. It will retract from its shell and filter feed.
Look at the very center of this picture. This is some type of barnacle. Any idea of what exactly this is? Will it survive?
thanks!

 

[speg]

Looks like you identified it yourself

 

[bornagain]

Over 1300 species, and I can't name one! They also have many life stages. This one seems too have a hard outer shell.
This rock is Caribbean. The Barnacle is about 2cm wide and 1 cm tall.
Anyone know anything about these? Have these in your tank?