Zerilium
From Flash Gordon Wiki
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's poisons into the atmosphere, causing the Great Sorrow. Soon after, the planet became uninhabitable.
The rift generator that Rankol built was powered by zerilium.
Speculation
Mongo's red-tinted atmosphere.
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.
