I am not always that good at identifying them but...I am going to guess O. hummelincki from length of arms. She looks similar to my Poe. Can you estimate size? Mantle and arm length?
Oh no, new tank? I assume it is not cycled yet. That octopus IMO won't make it through a hard cycle. I'd work on finding it a new home. Do you have another tank (cycled) to move it to? Maybe a friend or LFS. It would be a shame to watch it die slowly. That is an amazing hitchhiker.
Third vote for O.hummelincki. If it is female, we have seen high brooding rates after about 2 weeks in an aquarium. However, this may simply be because they are out foraging before brooding and more easily caught. Since this one is a hitchhiker the 2 weeks may not apply. It may also be a male and we have not seen any short keeping periods with the few males we see.
I particularly mention this because your fish will be in jeopardy at some point. O.hummelincki males make a particularly interactive pet but we don't have much data on them with fish since most successful keepers order them and prepare a species only tank. There are various observations with fish and octopuses but the safest for both is to not mix them. CaptFish did a very nice article on the growing dangers of fish with O.briareus (a larger species) but we don't have similar formal observations with O. hummelincki. If you lose one fish the rest will likely follow.
There is also a concern with the fish picking on the octo and damaging the skin or eyes. We have several journals recording fish aggression but only hearsay on permanent damage (the recorded fish aggression caused the fish to be removed before any problems occurred)
My female O. hummelincki was quite the tenacious hunter. She seemed to have very good eyesight. I'd worry about her hunting your fish too.
Do you have a secure top on your aquarium? Just in case she decides to go exploring.