emerald... upside down


One of my emerald crabs has been acting reeeeally weird the last few days. I noticed it upside down right up front in the middle of the tank maybe 4 or 5 days ago. I thought it was dead, but when I looked closer it was still moving it's legs, pincers & opening & closing its eyes. So I flipped him over. He walked about an inch to some food that had made it to the bottom during feeding, ate the whole piece very slowly, and just stayed there. A little while later, he was upside down again. I flipped him back over and about an hour later he was slowly climbing up the bottom of one of my rocks. I saw my cleaner shrimp walk past and knock him off and once again he was upside down in the sand. Several times since then I've flipped him over and he always ends up upside down again. He's still moving, still blinking, still eating a little... I don't get it.
Does anyone have any idea what's wrong with him? Nothing else is acting out of the ordinary at all. I have 4 other emeralds & a porcelain crab, as well as 7 shrimp & some hermits and everything else is great.