Nassarius snails are very useful in aquariums because they eat detritus, or waste, and keep the sand bed stirred. They do not eat much in the way of algae, but are an important part of any reef aquarium. They will also eat any excess food that the fish do not eat. Nassarius Snails stir the sand bed as they borrow through it foraging for food.
We have witnessed them going after some clams on occasion. I recommend placing the clam on a rock or on a flat piece of putty that protects the opening at the bottom of the shell. This way the Nassarius Snails will have a harder time getting at the delicate underside of the clam. Another option is to forget the Nassarius Snail and use Sand Sifting Starfish instead as they will do a similar cleaning job and don't bother clams.
Nassarius Snails are scavengers and will eat detritus and left over meaty foods such as mysis shrimp.