You can't put an octopus in with all corals, only with ones who a) don't sting and b) tolerate low light levels and remember every living thing you add to the tank increases the bioload! so be careful how much you add!
Live rock is basically fossilized coral with lots of living organisms inside and out. It contains bacteria that is extremely beneficial to your filtration. Quality live rock is very porous and lightweight, and will bring lots of surprises with it. Some good, some bad. Copepods, amphipods, isopods, macro and micro algaes, sometimes corals, sometimes crabs, shrimp, and on occasion octopuses.