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I've seen something like this before, these look very much like anemone lesions (the white patches will be scar tissue where the lesions heal over). We NEVER have anemones in our octopus tank (well OK we had some last month, came in on some new rocks, they are gone now) and yes too much of the trauma can kill.
You never mentioned how old the animal is. If it is close to senescence lesions will be much harder to heal and scar tissue will appear from rubbing against tank walls/rocks (often called butt burn!)
Yes the last one had some lesions on its mantel that looked identical to the 'butt burn' on a cuttlefish but my supervisor told me something different caused it that I thought wasnt the case. I'll tell my director about the anemones again but I doubt he'll let me remove them. He said that they are found in the same habitat as the octopus but I don't think he realizes that in the wild they can get away from them.