What's your PAR where the clam was? I don't know the Kessils well, but my guess is it's pretty low for clams - think moderate to high SPS level of lighting. If it's directly under a spot and they're turned up it could be sufficient, but in my experience with similar wattage lights, that's probably too low for anything mid-tank or lower. PAR numbers would give you the verification, though.
If not a fish nipping it, I'm not sure how likely a tear really is - if there was something that punctured it that wasn't a fish, it likely would have been closed on, so that there's a hole and that its open probably means it no longer has the ability to keep itself closed. Especially if the flesh has lost some color, the hole could actually be an early stage of necrosis, since there's basically no mantle extension and it looks sort of slack open.