Wow, it's really sunken in there.
I hate to say it, but if it's on its way out, .. well 1) there's probaby not much you can do about it at this point, and 2) you'll know before too long - IME it doesn't take long for a clam to go from "looking fine" to "hey, what's going on here?" to "hey, two perfectly clean empty shells! Argh!".
But, sometimes, I've noticed with mine, they pull in for a day or two. When my new squamosa spawned, it looked like that for about a day or two afterwards. Looks totally fine now.
So maybe it's something like that.. Good luck, I'm hoping for the best for you... !!