What is this?

[nvierra]

Is it a coral, or parasite? They rush inside their housing when you get close

 

[Catawba_Valley_Reef]

Is it a coral, or parasite? They rush inside their housing when you get close

Looks like a feather duster which is harmless.

 

[Mebbid]

Barnacle maybe?

 

[Mr. Mojo Rising]

Some kind of fan worm or tube worm

 

[Gumbies R Us]

@ISpeakForTheSeas

 

[ISpeakForTheSeas]

Barnacle maybe?

@ISpeakForTheSeas

Barnacle would be my first thought too, but if it's not a barnacle then it's a filter-feeding worm - either way, it's harmless and won't hurt the hermit crab:

If it fans out then rakes in like fingers closing into a fist, then it's a barnacle. If it just sends out the little crown of feathery appendages and basically sits with them fanned out in a circle, it's a feather duster worm. Either way, it's a harmless filter feeder.

Assuming it sends out a little clump of feathery appendages that fan out like a rake, then close in like fingers closing into a fist, I'd guess that's a coral-boring barnacle.