Green & Green is a guild that originated from Mamare's notes, and are never referenced in official texts. They are, at best, quasi-canon.
Green & Green is a massive international pacifist guild based in Perth, Australia. They seem to be the Round Table of the Oceanic Server, acknowledging that Landers have rights, kicking out PKs or anyone that harasses Landers. Green & Green is also clearing out monsters so they can work towards finding a way home.
They seem to have developed an Overskill or special technique called Art of the Whale Song, trying to tap into whale vocalization to communicate across servers.