The heroes came to a window and discovered, to their dismay, that they were being held captive on the Moon.
The Moon December 1, 19:36, USTTime Zone
The heroes had difficulty getting their bearings in the crystalline corridors. Captain Marvel came up with a plan to find the teleporters that brought them to the Moon, but the next moment the group was teleported inside a cell. They were addressed by Orb-One, providing translation for First Leader Kylstar, who told them they would remain where they were during their journey. Kylstar's ship then lifted off from the surface of the Moon and headed into space.
Background in other media
In the comics, the Justice League had their base on the moon until is was destroyed in Infinite Crisis.
For his previous series Gargoyles, Greg Weisman admitted the moon was always portrayed as full unless it was important to the plot. This seems to be the case for Young Justice too, as to date, only "Schooled", "Insecurity" and "True Colors" showed non-full moons.