Hey triggerSD, if they look like a whelk, you ought to be concerned. Best to isolate them and establish what they are before trusting them with your clams.
Myself and many others are happy to assist but your description alone makes it impossible to even guess. There are indeed whelks with white bodies but this alone is not enough to confirm your suspicion.
Your best bet is to ask Dr. Ron. Read his "post rules," particularly those specific to snail id's. In the end, it will cut to the chase and best assist you.
If you can't get a picture, a handy and easily obtained Peterson Guide, "A Field Guide to Pacific Coast Shells Including shells of Hawaii and the Gulf of California" by Percy A. Morris may also help.
(Almost every library has a copy.)
Cheers,
Bob