Mucktopus,
I think you nailed it when your wrote: "Both mimics and Wunderpus are hard to find". This does not always equate to "rare". I read a blurb on a National Geographic web site that said the reason people think that mimics are rare is that they look for them in sand/muck habitats near reefs, and their peak density is actually in brackish estuaries where nobody dives, or if you did, the visibility would be too poor to see them. Don't know if this is true or not, just offering it out there, FWIW.
Brackish octopus? I don't know. Besides, if the vis is so poor, why would the animal rely on Batesian (or is it Mullerian?) mimicry?
Msby 12 - the pale coloration is just how they look when they are not in their aposematic color mode. Also, I haven't seen that particular exhibit, but I don't recall seeing any five gallon exhibits when I visited LBAOP. Was it near the jewel tanks that housed their Rhinopias scorpionfish? Maybe the tank just looked that small in comparison to their primary exhibits?
Here is another thought - counter intuitive perhaps, but it is seen in Giant Pacific Octopus. If you give a GPO a large tank, it can more easily jet, and pick up enough steam to really whack into the side of the tank. Keeping them in smaller tanks (really just the size of their den) keeps them from being able to get up as much speed when they jet.
Same thing with zebras in zoos - either keep them in small paddocks, or in huge open areas (where they can run, but have time to slow down again). Keep them in a medium size pen and they'll build up just enough speed to hit the wall and break a leg!
Jay Hemdal