Actually, Mama does know perfectly well what his name is because Dee has introduced him by name. Using his greeting instead of his name is Mama's subtle way of commenting on Dee's "new" lifestyle and choices, including her companion. It is her way of saying she does not approve, even, perhaps, making fun of both of them as a bit pretentious.
One of the themes of this story is that one should be faithful to one's past and one's roots by living them, not by trying to adopt a false past. Dee's new boyfriend is unlikely to be from a family of the Islam faith, probably having adopted this new religion because, during this period, there was a movement afoot among African-Americans to adopt a religion that was not the Christianity imposed upon them during slavery, and because Islam was more strongly associated with Africa. Mama is impatient with this because it is certainly not Dee's "legitimate" past nor that of her boyfriend. It is clear as this conversation goes on that Wangero thinks little of Mama, either, and she comments that he was
...looking down on me like somebody inspecting a Model A car.
As the story continues, the differences between Mama and Maggie and Dee and her companion play out in the same way, with Mama and Maggie wanting to celebrate their roots by using their artifacts and embracing their true past, unlike Dee and Wangero, who want to use artifacts to show what they no longer are, but of course, no longer being true to their past at all.