Even though the link won't work for me, based on the picture, I'd say it's a Mimic Octopus, like you said. It basically tries to imitate any animal in the ocean that would conceal it from danger. I've seen them mimic crabs, cuttlefish, and many other things. This isn't an octopus in the hobby though. I don't think you should get your hopes up. The only octopi in the hobby are the Blue Ringed Octopus (which are very dangerous) and the Octopus named Thomas (don't know what kind of octopus it is). In any case, all octopi are dangerous and their life span's only about a year in captivity so it's not really worth it, especially since if you don't keep it entertained, it will try to escape the tank.
