Watch him to watch for blockages, but most likely he's ok.
I accidentally found out my marble cat will eat asian clams. He apparently digests the meat and then regurgitates the shell. I think my dat ate some too, and he's ok. If it's one of the thin shelled clams, it will probably break down during digestion also.
He actually may have spit the shell out at some point and you didn't notice it. My dat spits out shrimp and crayfish shell after awhile.