Throw it out.
Clam at this stage is almost as good as dead. The mantle is already starting to detach from the shell. Next step is decay and you'll won't want that in your tank.
I had one of these as well and that is what it looked like right before it died. Sorry for the bad news. I got mine at CMAC and it lasted about 5 months and just suddenly did that and died a few days later. After losing it, I did a bunch of research and it seems many people have a hard time getting the Tahitian Maxima's to live more than a year in captivity. There are very long threads about it on RC. I was very bummed as they are so beautiful. I almost bought another one at FFM but just didn't want to it die again.
good news it didn't smell !!
I got a year and a half from my last Maxima.Under halides and stable water conditions.Just died for no reason one day.