قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ : " إِنَّمَا سُمِّيَ الْخَضِرُ لأَنَّهُ جَلَسَ عَلَى فَرْوَةٍ بَيْضَاءَ، فَإِذَا هِيَ تَهْتَزُّ تَحْتَهُ خَضْرَاءَ "
And the Messenger of Allah, peace and blessings be upon him, said: "Khidr was named «خضر» because once he sat on white sand or dry land, and suddenly that land beneath him turned lush and green, waving with vegetation."
Related hadith on this topic