Dwarf Cerith has white fin?

[Neocold]

Anyone know what's going on with this Dwarf Cerith snail? It's had this white fin for a few days at least. It moves all around the tank with it.

It's in my 5 gallon invert/coral quarantine tank with about 31 days left on this quarantine cycle.

Thanks in advance!

 

[betareef]

Looks like something (a oyster shell or sponge?) is attached and growing on the snail shell.

 

[Neocold]

Finally got a couple pics of the underside. Wish it was better quality. It dropped down to the sand before I could get a better one.