What is this hitchhiker?

[SopmodSnapper]

Just got a shipment in, and in a freebie bag, was a coral and this slug??? It's about the size of a No.2 pencil eraser. Anyone have a better idea of what this could be and if I should add it to my tank?
TIA

 

[ISpeakForTheSeas]

Stomatella - good CUC; it's a keeper.

 

[SopmodSnapper]

Stomatella - good CUC; it's a keeper.

Awesome.... in the tank he goes.
Thanks