Maxima clam retracted mantle

[wii64brawl]

Not really sure what to make of this. The clam has been happy for almost four months now, but I just noticed yesterday that part of his mantle is retracted. These pics were taken last night and really no change today. Certainly doesn't look like PM, but I really have no idea what else it could be. He still reacts well and besides this part of his mantle, expands well. My other two derasas have shown no symptoms of PM either, leading me to believe this guy is possibly being irritated by something. Yet on that side of the shell, there is just some rock. I stared at him for about an hour last night, no parasitic snails, nothing picking on him, and I couldn't see anything inside the shell that could be irritating him. I'm really at a loss. Anyone ever seen anything like this before?

[ReefWeeds]

Has it always been on it's side like that? Was there anything near that side of the mantel that could've stung it? Mine did that and when I looked, there was an aiptasia nearby.