Mollusk on hermit shell?

[ReefSparty]

I’ve seen this calcified mass growing on hermit shell since Got him over a month ago. Today I noticed a soft black body poking out the base that retracts when startled. Doesn’t seem to bother the hermit but is this something I should be worried about?

 

[Reefer Reboot]

Jingle shells, a type of bivalve, nothing to worry about.

 

[Rtaylor]

Slipper limpet?

 

[WheatToast]

Hoof snail? These are harmless, sessile grazers.

 

[ISpeakForTheSeas]

Hoof snail? These are harmless, sessile grazers.

I’m inclined to agree with it being a hoof snail (taxonomic family Hipponicidae). Some of the species you’d find on things living in shells feed on the waste of their host, others basically feed on whatever is brought into their range by their host. Harmless, sessile, and a neat find.