I am fairly confident in saying it's a lost cause. Did you look for the presence of Pyramid snails, they are extremely common? Clams can die suddenly. I am not familiar with your lighting type. What is left of of the mantle does not look like PMD. If this was a clam under 2" it's not unusual at all for them to suddenly die. Looks like a Maxima, what type of clam is it? Have you recently added new carbon or gfo?