How big is he? Because small or teeny crabs can molt alot, really fast. My teeny, George, has molted four times since August. His shortest one being less then a week, his longest being three weeks, and now he's a small!
Have you checked for needle feet? Or was he displaying molting signs before this happened? (ashy exo, excessive drinking/eating) He might not be molting at all, he might just like it underground! I know some people, me included, have crabs that almost never surface, but they aren't molting, they're just being little stinkers
![Very Happy :D](./images/smilies/003.gif)
Another possibility is that he's trying out different spots and different tunnels before he decides which one is the best to molt in
![Smile :)](./images/smilies/icon_smile.gif)