It is yet to be proven scientifically, whether the mimic octopus is a poisonous creature or not. The survival strategy of camouflaging itself might suggest that it is not. Scientists and zoologists believe that if the creature were poisonous, it wouldn't have had to hide itself from its predators. It would have just attacked them with its poison.
Read more at Buzzle: http://www.buzzle.com/articles/amazing-facts-about-the-mimic-octopus.html