What is the meaning of Maymun's saying about Al-Hajjaj: "A Haruri, an Azraqi"?

Maymūn ibn Mihrān clarified that his statement meant it is permissible to pray behind someone who is known to be a Khārijī, because the worshipper prays to Allah, not to the one he prays behind. Indeed, they used to pray behind al-Hajjāj al-Thaqafī, despite him being a Harūrī Azraqī who deemed lawful the shedding of blood of anyone who opposed his view.