For starters, the Xarax-hil have no bioluminescence, so they don't glow. What you saw for Endy was an aberrant pelt coloration, uncommon in the same sense that someone can have facial hair of a color different than that of their head hair.
Second, while I have a working theory of Errux's geological history, I have elected to keep that history to myself for potential discovery in future works.
Third, as Kaerwyn continues, more opportunities to know about the Xarax-hil may arise, so simply keep reading.