What an interesting question. Of course I did a google search on "Octopus macropus" + predators and found a ton of references there. However, the greatest majority allude to the colour pattern of macropus having something to do with predator avoidance .... but none (that's not to say nothing exists) actually detailed these predators.
Goes to show the limitations of google and/or science.
Chejjjj(+1), I suggest you do the same google search; I am afraid that you might have to wade through quite a bit before you find what you are looking for.
Although if you click here you'll find Risso's dolphin eating this species.