We got two big rocks with lots of hiding places, standered biocube filteration ( with skimmer, of course ), and emerald crab that is a little bigger than the octo and a slightly smaller fiddler for it to eat. I've got 20 or so fiddlers in a bucket, and we introduced it by letting it climb in. And we have a 29 gallon biocube.
It is likely hiding in the rock (they are nocturnal and hide with the lights on) but check the sump as it is often a place they find inviting and the grid gives them access.
I am confused about the emerald crab though. It is bigger than the octopus? Emeralds don't get much bigger than the first joint on a pinky finger. If the fiddler is a male, remove or disable its "fiddler" claw (the smaller claw and the claws of females are not usually a problem).
It is a little bigger than the octopus, and we checked the sump and the fiddler is pretty small but if it's ABSOLUTELY POSITIVELY NEEDED than I guess I could take it out and fiddle with the fiddler I REALLY HATE touching them though. It feels pretty weird.