It's unfortunately a fact of life with cuttles and octos. That's partly why not many people keep them, harder to care for and they don't live anywhere near as long as most fish in aquaria.
There was a guy on youtube with an octo (bimac or vulgaris, I don't remember) and he had some videos of it, very interesting, and he had a video of it near the end of it's life. Completely white and barely moving, very hard to watch.