I am most worried about stress and not being able to hide. It is their nature to stay hidden most of (if not all) the time until somewhere between 4 and 5 months. My guess is this is either approaching sexual maturity or the size where they become predator vs prey. The den needs to provide a hiding place and complete darkness. Something that spirals inward might be best (like a snail or small conch shell). This will give a varied amount of room, allow you to place food where it will slide toward the animal and give it security. I used shells for the mercs but the ever escaping briareus seemed to prefer the live rock. Without a choice, however, the shell should be accepted.