Imam al-Bukhari relates that the Prophet Muhammad (peace & blessings upon him) said, “Khidr was named ‘The Green Man’ because he once sat on barren white land, after which it turned luxuriantly green with vegetation.”
The important role of Khidr as the initiator of Saints may be illustrated by his role as the initiator of Prophets, particularly of the Prophet Moses. Even with the depth of knowledge of Moses, who was one of the five great Prophets sent to this world, God caused him to be in need of Khidr, even though Khidr was not a Prophet (related in the Quran 18:60-82). This is to teach us, as God says in the Holy Quran, that “Above every knower, there is a greater knower” (12:76).
Khidr has two special gifts from God – the first, His mercy, and the second, knowledge from God’s own presence.
May God be well pleased with him.
Source:- “The Naqshbandi Way – History and Guidebook of the Saints of the Golden Chain”, by Mawlana Shaykh Hisham.