I would check for pests, pyramid snails, vermitid snails on the shell, aiptasia, etc... Also, that striped hermit crab is a no-no in a reef, especially clams. I wouldn't say PMD unless the mantle is recessing more each day? If it's just that lobe of the mantle, I would look for something irritating it. Also, feed the tank, not the clam. Feeding clams is unnecessary, but providing them with nitrogen via phyto is okay. I would increase lighting intensity to achieve 250+ micro moles, 150 isn't going to cut it for very long. Just make sure you're seeing new shell growth underneath the mantle's upper margins. I would find a spot on the rockwork higher up, which is where maximas should be placed anyway.