The term "Reef Safe" is banded around a lot and it's general meaning is sometimes nisunderstood.
I've seen people complain that their "Reef Safe" lobster killed their fish, it's a predator... it eats fish. That does not make it any more or less reef safe.
The way I understand it is that reef safe means that it will not harm a reef tank, but meaning reef as in the invert. sense. While an Octo won't eat corals which some butterflyfish/puffers might they can damage them by climbing all over them and they can too also be stung by some corals of, for example the genus Euphylia.
Also beware that bimacs love eating bivalves such as clams... and if you have a good £50+ maxima the last thing you would want to do is to add an octo and watch it get munched.
Totally agree with everything people have said above though, just thought i'd add my ten pence
I'd also think that a 250 watt MH would be too much for an octo. You might find that they will just and you'd probably not see much from them. You would however need that lighting if you wanted to keep SPS corals!
~Andy