Yes, they are pyramid snails and they will eat your clam alive before your eyes.
Something like a six-line or canary wrasse will help some, but the most effective way is to simply remove the clam and scrub/pick them off AFTER lights out (they're nocturnal). Do this every couple of days for about a month.
They like to attack the clam's byssal opening on the bottom so protecting it by having the clam off the sandbed and firmly holding onto a small piece of rock is a good idea.
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