This page of the Elex 2 guide contains a walkthrough of the Purple is the New Blue quest received from Caja after completing the Securing the Land quest. The mission involves you helping Caja collect samples from the Skyanids.
Collecting samples from Skyanids
After you receive the quests Caja will notify you from time to time about locating the Skyanids you must collect some samples from. She'll do it five times in total but you must do the same thing each time. You'll be teleported near your objective and tasked with eliminating all nearby enemies. Then approach the purple crystals Caja will start examining them.
The third trip is worth singling out, as it's the most difficult one. You'll encounter a Behemoth with a staggering amount of health. It deals really heavy damage as well. You must defeat it to continue - Caja won't start her examination otherwise. If you're unable to beat the Behemoth you'll have to return to the task later on after upgrading your character.
During the final trip you'll visit an area full of Skyanids but you don't have to fight them - defeat the enemies in the vicinity of the purple crystals and Caja will examine the samples. After the fifth trip you'll complete the quest.