I also order small live saltwater crabs (not fiddlers) from Kevin, at_your_door_fishstore. If he does not show crabs listed, send him a message through eBay and mention me . He will know what kind of food to send.
In the wild, it's pretty common (if not universal?) for members of Callistoctopus to bury in the sand/rubble rather than entering into a solid den. Give a pile of pebbles and some sand and it go into the corner and pile a bunch of sand/rubble over itself each morning, and come out all night.
That type of activity is not uncommon for octopuses in aquaria. Is it a male? As it's facing away from you, check the arms on the right side. If the third one back is much shorter than the opposite arm (i.e. the third left arm) then it's probably a male. Males sometimes pace a lot, and this might be related to aging, and/or mate searching behavior. In the wild a full grown C aspolosomatis could wander for easily 50 ++ meters a night during its foraging bouts, so it's not a surprise if they seem not to settle into a tank.