Zeks (aka Little Green Men or LGM) are small (less than 1m tall) humanoid creatures on Mars who are tasked with erecting hundreds of stone statues around the sea. Ilium proposes an origin for the term:
The word meant nothing to Mahnmut. He felt the LGM’s communication organ pulse in his hand and had the wild urge to release it, to rip his fist from this doomed person’s chest—but that would help neither of them now. Do you know this word—‘Zeks’? Mahnmut asked Orphu.
Just a minute, sent Orphu. Accessing third-tier memory. Here it is — from A Day in the Life of Ivan Denisovich. It was a slang term related to the Russian word “sharashka” — “a special scientific or technical institute staffed with prisoners” — the prisoners of these Soviet labor camps were called zeks.