Haven't seen my cleaner do that, but is it possible the snail died and he's just cleaning up? Nassarius snails will usually be in the sand and only come up for food so if they were on top of the sand, I'd suspect they were already dead.
I saw the Fire Shrimp twice pull the snail out of the sand and rip it out of its shell. Both snails appeared to be alive at the time. Seems like this may be specific to this individual and not to all fire shrimps.
What a cool shrimp sounds like a headache for you though so ill just take him off your hands. lmk when to pick up i have a barebottom so it wont be a problem