I just found this online, so I guess the guy is okay.
Safe shrimps and crabs:
Symbiotic shrimp from the family Palaemondidae (Anchistus, Conchodytes and Paranchistus) or small crabs such as Pea Crabs of the family Pinnotheridae may be visible through the inhalant siphon in larger clams.
These animals live inside the clam and do not harm it, although what they eat and what they do for the clam is unknown.