I guess being an Monkey's Uncle and have a 1 in million clam, it appears to be still a live, but clearly not doing well.
View attachment 110637
View attachment 110637
That is a rock boring clam that need exceptional amounts of light. Find the hottest spot in your tank on a rock and get it situated there. Keep your nutrients between SPS and LPS levels and make sure nothing is bothering it. If it can fix its byssal threads, it should stick its mantle out and hopefully start growing some shell.
I don't see any new shell growth on that clam, which means it might already be on its way out. People, correct me if I'm wrong.