No sign of him until tonight. I added 4 live feeder shrimp (Palaemonetes vulgaris) to the tank on tuesday. It didn't seem as though he was successfully feeding on them though, as they were all accounted for since then. I didn't get a conclusive count today, but it seemed like he *might* have gotten one last night.
A few minutes ago, I saw tentacles sweeping from a rock, so I sat and watched. He unsuccessfully swiped for a shrimp. I went ahead and captured one from the container where I'm gutloading them and came back to offer it to him. When I got back, I found him crawling around in the chaetomorpha ball in the corner of the tank. I offered him the freshly killed shrimp. He needed no convincing. He took the shrimp, ran to his hole and that was that.
I'm wondering if he's been foraging in the chaetomorpha. The tank is effectively a refugium. I have a big chunk of chaeto and a small forest of Caulerpa prolifera. The sand is ~2" deep and heavily seeded with worms of all kinds and pods. The chaeto is teeming with copepods, isopods, amphipods and mysis shrimp. Is it possible/likely that he's living off of pods and mysis shrimp?? He is quite tiny - maybe 1" mantle and ~3" total legspan, if i had to estimate. He seemed to be eating hermit crabs for a while, but I think he's mostly depleted the supply of them now. The microinvertebrates, however, are quite plentiful.