Whew, glad your octo is OK. Steve is right, proabably just stressed for a little while.
How big is your octo - do you think it's an adult, or younger?
Take out everything but the snails and crabs (unless they are large). Take out the sea hare, too. (That's unusual here, have never seen one in an aquarium store, just read about them.) If the fish are smaller, he will eat them, if they are larger, they can bother the octo and stress him. This is different from the ocean - they can't really get away from each other. He will probably eat the crabs and maybe the snails. Do you have another tank to put them in?
Good luck with keeping your octo!
Nancy