What in the world is this?

[GhostCon1]

The electric blue things with the black in the edge of the square

 

[Whiterabbitrage]

.??
Can you take a closer picture? Does it move at all?

 

[GhostCon1]

That's the closest I can get.
Yes, it retreats into the hole somewhat.

 

[Calappidae]

I'm following as I'm curious too..

 

[GhostCon1]

We've narrowed it down to either a coral crab, which I don't think it is because of the fact that it's lacking the red claws.

Or a bivalve of some sort.

Also I'm thinking maybe a polychaete worm.

(Edit: wow I cannot believe that Siri knew how to spell that)

 

[OoNickoC]

Definitely a boring bivalve. We got some in on a shipment at a store I used to work at. Ours were purple and black though, that one is gorgeous. Harmless to the coral as the don't grow quickly and spread much.