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.
Upon 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". The clue to it is a small recorder on a table at the dorms in the Desert Island Base. Answering incorrectly only makes the outside guards hostile, and they're not too hard to take out.