I have a theory about the "mimic". I am not sure it does anything that other octopuses don't do. Just the short time observing my "simpleton" pygmy, I noticed it emulating (or I had the impression it was) other things. The mercatoris is not known for anything special but I definitely thought it was several other things (one being a large crab and I knew I did not have one of those in the tank!). The emulation under red light was very convincing and I am sure not accidental. In the case of the "crab" the movements were very crab like and enough to make me do a double take. I also saw her repeatedly act and look like a snail and a fish. Had those creatures in the same size been in the tank, I would not have realize it was the octopus. Unfortunately, I don't have a camera that could witness this but I am certain the posturing was deliberate and had nothing to do with the 3:00 AM and 4:00 AM observation
I think the excitement over the mimic's ability may have more to do with its coloration and unique looks, giving it the ability to look more like the creatures it emulates in the daylight to the human eye.