The Moon Phase is one of Unturned's game mechanics.
There are five moon phases in this game currently: Full Moon, Quarter Right, Crescent Right, Crescent Left and Quarter Left.
When this happens, all Zombies gain red-tinted glowing eyes, including the Mega Zombies. They deal more damage to Players and have a much larger detection range. The Zombies also respawn much faster. Zombies killed in the full moon will yield double experience.
An easy way to avoid a full moon is to quit the map, then re-enter it. You will then see that the full moon will have disappeared and will instead follow its usual phases.