LOL, that IS his den. More than one keeper has been frustrated by their octopus ignoring cleverly sculpted dens and choosing some other humanly unaesthetic hidey hole. Using or creating a door is generally more frequent when they are nervous withe their surroundings or brooding. Rearranging rocks is another favorite of some animals but I have only seen major rock shifting when a female is securing a brood den. Others have seen males do it though and sometimes more than once either out of boredom, growing or some other unknown reasoning. The one thing I THINK I get is that most (my female mercs have been an exception) want a dark place where you can't see them.