Nubium is one of the pair of evolutionary gods, two extra-dimensional gods who work together to create worlds. The moon worn by Nubium is one of the actual moons of one of their planets, much in the same way that Tethys' planet is actually one of their planets, and the pair can switch them out for their other systems.
Nubium is a hard working God, but both he and Tethys have to abide by all the laws of the universe. They can't just "poof" a planet into existence. They can however travel through time and visit the planet outside of chronological sequence.