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Crab body in the last one? Try to observe how he reacts with some of your anemones. Octopuses, particularly O. briareus, don't go around things and stings are an infection concern. If you see him touch one and withdraw, it would be wise to remove that animal from the tank. Leathers, gorgonians and mushrooms are better suited than most polyps and anemones. In the ocean they can avoid some of the issues (or just get sick and become food for other animals) but in a tank, anything in their restricted hunting ground is moer impactive. Keep in mind that when stinging cells are released in the water, they are not scattered quickly like in the ocean so extra care with what we place in with octos is warranted.
Sadly, I am afraid ghost may be older than I originally hoped. That "hiccup" action, the prominence of the eyes and the way his mantle is kind of loose and not firm are all signs that he is aging. I noticed the eyes before but, as I mentioned, stills don't give as much information as a longer observation. With good water, this species seems to go through a longer senescent period (or start earlier) than the smaller animals so I don't expect death to be immediate. Animals are often a bit more active during this period so don't hesitate to interact. I have often wondered if their skin gets "itchy" and may be the reason they seem to enjoy being "petted" once they know you are not interested in eating them.