I'm so sorry it's taking me until now to get to this. I had a holiday party this evening.
That's one unhappy clam!
It really does look like it's being nipped at, is this the tank that also has the flame angel in it? I have found that my flame angel will nip at my clam every now and then. When I catch her, I chase her around my tank like mad with the net and that seems to make her stop for awhile until she gets up some courage again. That being said, you've had problems with the tangs and smaller clams, so it could very well be Ms. Sailfin making an attempt to pick this one up and take it for a ride as well. Remember what I just went though with my Sailfin. Ms. Coral Eating Sailfin took a trip to the LFS and is now in their fish only display tank before I couldn't deal with it anymore. I still think that she was the one who ate my last Maxima clam and I think you are experiencing the same thing with yours now. Why can't these darn fish just behave themselves?
I do think it can recover, BUT it won't if it continues to be irritated. Honestly, I don't like the way it looks right now, but not all hope is lost and I truly do believe it can heal. I know you moved the last one to a smaller tank, any way you can move this one as well?
The net is an okay solution for the moment, but either it needs to be removed from the tank, or what's bothering it does. Since you don't have an LFS down the street to give your naughty fish to, I think you're going to have to move this clam to a different tank. The net is only going to irritate it more.