Id - thing on my hermit crab's shell

[lastlight]

So I suspect this hermet of snacking on my poor birdsnest (which is looking even worse now) and I had forgot about two of these things that live ON his shell. They are under these mini shell type barnacle thingies. They reach out to feed but their little shells don't seem to move from their spots.

Can these things snack on my sps if the crab is camped nearby? What are they? Thanks!