I can't tell 100% by the pic as some look like sand but there are resemblances to pyramid snails. Also check under the clam as well if you can. We just lost our big squamosa due to those snails. If he's not attached then a light brushing with a soft toothbrush in a separate bucket would be a good idea.
OK, so I just brought him out and gave him a light scrubbing. It didn't appear that any of those were snails - more like collection of sand. The only thing of interest I could find was this worm. Looks kinda like a bristle worm, but not sure - it's kind of a wiggly swimmer when in the open water. It could have come out with the piece of rubble that the clam is attached to, but not sure... Thoughts? Keep or cull?