After hop after hop after hop, I decided to try to box-in my clam. Got some rock from the sump, and ended up with this:
I also lowered my main lights a bit, since the clam is so low in the tank. I wish I knew what kind of PAR the LED fixture is delivering.
The clam looks ok still, although there is a section of mantle that is pulled in a bit - right front, in the picture below. Hoping that's not indicative of a problem.