Yeah, that's a murex snail - its diet is most likely bivalves (clams, oysters, mussels, etc.) and/or other snails; so if you have any CUC snails or hitchhiking bivalves, I'd sump him ASAP.
Neat snail though, and you may be able to keep it by offering things like grocery store clams or frozen sea snails - I know babylonian snails and periwinkles are relatively easy to find.