He will eat shrimp, crabs, and snails. He will also eat any clams in the tank. Make sure you have a securely locked top so he can't escape. He shouldn't be poisonous. The only way the poison will hurt you if he does turn out to be poisonous is by him biting you. He may ink if he gets spooked. If he inks try to net as much as you can and make sure your skimmer and other filteration is on top capacity. He may also go after fish in your system, but he looks small, so he shouldn't go after big ones. You may want to consider moving him to a different tank if you have one, or if there is room in the sump somewhere for him he will probably be happy there. Octopuses do not like a lot of light so if possible decrease your photoperiod or move him elsewhere.
Most of our experts are at a convention down in FL this weekend, so you probably won't get a positive id on him until the weekend is passed. Don't worry however if you don't see him for a while or if you hardly see him at all because they hide a lot when they are in a new tank and he is most likely nocturnal as well. If you run actinics, 14k, or 20k bulbs you will see less of him as well because the blue spectrum is very bright in the eyes of an octopus. For this reason it might also be a good idea to not run moonlights for a while if you have those too.