Howdy,
OK, this is just idle speculation subject to correction:
1. The parasite shown is of a group commonly referred to as "fish leeches".
2. The fish leeches I've seen have suction cups at either end for attachment and movement; a large one at the tail and a smaller one at the head that is equipped with a sort of hypodermic needle for sucking the host's blood.
3. These suction cups exert a surprisingly strong grip and tend to grasp onto whatever they contact. I've had to pull them off of fish, glass petri dishes, and my own skin and it seemed that the best adhesion was to the glass. I couldn't really feel anything from the one that stuck to my skin.
4. A bit of Googling didn't turn up any references to
external parasites on octopuses.
Speculation A: The leech in the image was more of a hitchhiker than a parasite on the octopus. It could have been picked up randomly as the octopus travelled about or the leech might have latched on because the octopus had just eaten its host.
Speculation B: At the time of the photo, the octopus simply hadn't noticed the leech. Eventually, either the octopus would find and remove it or the leech would leave on its own to find greener (bloodier?) pastures.
Speculation C: I'm not sure if this is the normal state of affairs but at least some of these leeches are obligate parasites on one species - I suspect that a leech that evolved to target an octopus would look quite different that the one pictured.
Speculatively yours,
Alex