The Green Planet is a scrapped location in Serious Sam: The First Encounter.
The Green Planet would presumptively be a planet filled with a lot of green wildlife. There are a few screenshots that match its description and don't seem to be set in Ancient Egypt, but there's no real way to determine if the location in them is actually the Green Planet.
It would consist of one level; Green. After finishing it, the player would be sent to the Ice Planet.
Unlike most of the scrapped planets, the Green Planet doesn't seem to have an Elemental enemy that would exactly fit it. The only Elemental enemy that doesn't have a planet that's a direct match for it is the Air Elemental, so its possible it would've been used there or at another planet without a specific Elemental enemy; the Tropic Planet.
Like all of the post-Egypt story arc content, the Green Planet was removed from the final game.