Rather than letting the Daxamite die on Earth from lead poisoning, Superboy sent him into the Phantom Zone to endure throughout the ages.
It was only when Superboy discovered that Mon-El was unaffected by Krytonite that he realized they were different races of aliens.
Unfortunately for Mon-El, he was affected by lead rather than Kryptonite, and Superboy accidentally exposed him to a lethal amount of the substance.
Lar Gand came across a young Clark Kent, then-Superboy, who wrongly assumed that he and the similarly powerful Lar Gand were brothers of the house of El, and that Lar Gand’s name should be Mon-El.