Going back and looking through the pics you posted, there are a couple things that stick out to me. One, it doesn't seem like that clam had been provided enough light for a while, there's no significant growth on the shell, so the retailer wasn't keeping it under optimal conditions. Two, the other animals you have in your tank are exhibiting growth forms that are similar to low/medium lighting. So you take a clam that's been in sub-optimal conditions and move it to another tank/system with the same or worse conditions and it's a downward spiral. Maxima and crocea are kinda tough as starter clams, as they need optimal conditions from the start. Provide them less than optimal conditions and you're simply fighting against time before their demise.