pyramidal snails will eat the mantle.
what is the' little grouping around the base of my clam' doing, is it visible during the day, do the snails move up after lights out, how many snails, what size? pic is necessary for positive i d.
you say, it looks very healthy and wide open. well that's b/c either the snails are NOT pyr snails [hope for your sake] or they have not yet done enough damage. pyr snails climb up when lights are out and retreat after morning.
to i d, look at the swirling on the shell from the tiny tip upwards, if the swirl is perpendicular, not angled, to the tip, then chances are, you need to treat the clam and fast. i can send a link for this if you pm me. or jared or william above can forward it to you.