Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Al-Khidr was named so because he sat over a barren white land, it turned green with plantation after his sitting over it."
شرح الحديث :
The Prophet (may Allah's peace and blessings be upon him) says that Al-Khidr got this name because once he sat on a barren piece of land that had no plantation, and it turned into a greenery. Al-Khidr is mentioned in the story of Mūsa in Sūrat al-Kahf. Scholars differed whether he was a prophet or a righteous ally of Allah. The sound opinion is that he is a prophet.