I should have figured that as octopuses are out of season here in the states for the most part.
I looked a little and there are a few options. Where did the rock come from? Did you order it from say, a place that would get it from south east asia? Or was this local harvest? If it was local there then it could be a few different choices...
Octopus aegina (Baby Octopus) though probably not because the webbing isnt really obvious.
Octopus tetricus (Sydney Octopus) Maybe-but nocturnal.
Octopus australia (Southern Octopus) not a blue ring from the looks of it.
Octopus maorum (Maori Octopus)
Octopus pallidus (Pale Octopus) have you seen if it has a number of sharp spikes of skin over the body which can be raised, along with a regular patchment of small raised oval patches over their entire body and arms? If you have seen anything like this then you could assume its the pale octopus.
Octopus graptus (Scribbled Night Octopus) couldnt find any info quickly on this guy, sorry.
If it came from South East Asia, then I could guess Abdophus spp. They are the more common species to come from that area.
How long have you had it and what are you feeding it? Also, it really needs to have some live rock with it as it will die from stress if you dont allow it to hide.
I wish I could get a hitch hiker like that.
