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The pointy things near the mantle look to be vermetid snails. Their mucous strands could be irritating the clam and not allowing it to open. I would look at water parameters too... could you post them? Also, have you moved the clam or changed lighting/flow recently? I don’t think it’s likely that it settled as a larva from the live rock you added. It must have been on the shell from the beginning when you got it.
Can you take a photo of the clam when it’s open to it’s maximum potential for the day? If he is this closed up throughout the majority of the day, it is likely a pinched mantle disease. In that case, your last hail marry is to do a proper freshwater dip with a matching ph and temperature. Otherwise, he is likely going to die in a few days. If the clam was close up like that because it reacted to something then you should be ok.I removed all the vermetid snail from it today. I don't think it's my water as my sps are growing so fast I have to cut them back every week and basically through them away.