Actually folks, I have some anecdotal findings that may surprise you!
I was researching using seahorse ponies as cephalopod hatchling food and though I haven't had any opportunities for experiments with inklings eating ponies, I can tell you with great confidence that all my adult octos ignored the adult seahorses. Three octopuses now have lived out their lives in peaceful harmony with the breeding pair of adult seahorses in a 90 gallong tank. That's two bimacs and a briareus.
In fact, the male seahorse had a hysterical habit of trying to hang on to the briareus, and the briareus wasn't terribly tolerant of this, but never so intolerant that the seahorse wound up on the menu.
There are a LOT of species of octopus though, so your mileage may vary. Just be careful.
Have kept adult seahorses with hatchling cuttlefish, worked well. (I was nervous as the cuttlefish were the same size as the shrimps both were eating!). Ended up with 3 cuttlefish, hundreds of seahorses. The seahorses sometimes being barged flat in the hunt for shrimps as the cuttlefish grew.
Did suspect that someone was eating some of the seahorse young before the system crashed with an ink-out.