I am sorry, but this little one is not likely to be alive come morning (and may be dead at the time of the photo, the chromataphores will function somewhat after death). If you see no breathing, she is gone. If you are not sure and want to euthanize or at least be sure, the best recommendations I have read are to place her in a cup of salt water and put it in the freezer but I suspect she is gone. I have done this only once and it was more my heart that was not sure and not signs from the octopus.
Unfortunately, size is not a good indicator for octopuses. Even octos from the same brood will grow differently (and it appears the same may be of cuttles as well), additionally, if she is post brood, she will have been living off her own body during the brood cycle.
This is the hardest part of keeping a ceph. It seems so unfair that they are so short lived and keepers become so easily attached.
As far as leaving her in the tank, your option. I have removed mine to small aerated containers to avoid having them eaten by my clean-up crew until they were fully dead. In a larger well cycled tank, there is little biologic impact from a small ceph (AM has seen negative results from a larger one though) but a small tank will likely spike if you leave her in there too long after death.