When I’ve known people with a nickname or stage name that they went by off-stage, you just called them that.
I actually can’t recall the last name of the guitarist for my last band. I’ve heard it twice, but I’ve never had to use it. We’ve used his actual first name to refer to him in the third person, but I’ve always addressed him as just “Raj”.
Apropos of nothing: My favorite nickname ever belonged to an acquaintance of my sisters, he went by “Rat Telephone”. I have no idea how he got the name,
His “recording contract” name is Prince. I think that’s one of the reasons he refused to let himself be called by that for a few years.
The credits to The Fresh Prince of Bel-Air calls Will Smith “Will Smith”, but under “theme song performed by,” it says, “The Fresh Prince.” Adele, Bono, and The Edge all use their non-stage names (Adele Adkins, Paul Hewson, David Evans) in their Oscar songwriting nominations. (Why Lady Gaga doesn’t, I don’t know.)