Emerald Moon Base is a secret base built by the Esmers on the Emerald Moon. It is built on the order of the Emperor of Zeelich with the goal to construct a gigantic jet engine for the Dark Monk's plan.
On arrival, the base's radio operator asks him the password. Twinsen needs to have a Translator to understand him. The correct password is "Operation Green Moon". There's no clue to it, the player has to guess. Guessing incorrectly only makes the outside guards hostile, and they're not too hard to take out.