Zerilium is a glowing red ore from Mongo's moon. In the time before the Great Sorrow, zerilium was mined from the moon, processed on the artificial moons Arkaylia and Surd, and then used as a power source on Mongo. Some time later, an accident happened on Mongo, releasing zerilium into the atmosphere, causing the Great Sorrow. Soon after, the planet became uninhabitable. Even generations after the planet was reclaimed, most of the water remained Grey water.
The atmosphere of present-day Mongo has a red tint to it. Zerilium could still be in the atmosphere, just at a lower percentage, allowing Mongo be inhabitable again.