Do you mean he is in the open all the time or that he has done no construction and sleeps in the liverock?
If you don't see your octopus taking a den to sleep this may not be a good sign. I hope I am wrong about this little guy but with the Mercatoris (behaviors are often noteably different from the larger octos), I have observed that when they stay out of their dens all the time it is the last week of their senscence (also an explanation of why so many WC die soon after arrival since they are easier to catch). If your fella is really Briarius, I fear this may be the case since they are naturally nocturnal and untrained, long duration, daytime activity would also suggest senscence. Since he is eating well, hopefully my thoughts are off base.
If, on the other hand, you are looking for an engineering feat and are disappointed that he just slips into the LR, this is not a concern. From memory (no quotable times) I seem to remember den building activity taking quite awhile for younger octopuses. Take a look at the thread on Biddle, I seem to remember Carol saying something about him/her being quite active lately with tank redesign and she has had him several months.