It is Makruh (disapproved)
to call each other by saying, "O mister so and so", for the Hadith of the Prophet (peace be upon him) indicating its disapproval. It is permissible to call someone by their names or their Kunyah (honorific starting with 'Abu' for men and 'Um' for women) or by saying, "O brother/sister" if they are Muslims.On the other hand,
it is better not to call anyone who performs Hajj by, "Hajj". This is because the reward for performing the obligatory acts is not names and titles but the reward of Allah (Exalted be He) to whoever He accepts their Hajj. A Muslim should not be attached to such titles as his intention should be for the sake of Allah (Exalted be He).May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.