if he is moes' guy, why does mike discuss this personal family business in front of him. surely mike wouldn't / shouldn't be this sloppy, or just directorial license?
Well, you can never rule out directorial license in any detail of the Trilogy that doesn't make sense. Plus, turtlenecks with sports jackets weren't a fashion trend until the late '60's, and that scene was set before 1955.
I'm guessing that the guy was hotel security, summoned by Fredo to make sure no one crashed the party he had planned for Michael with the girls and Angelo, the accordion player. He may not have specifically been Moe's guy--just an employee. You're right: Michael logically shouldn't have discussed business with him around, but maybe he just ignored the guy. Or maybe he figured the guy was paid to keep his mouth shut. Or maybe it was, as you said, directorial license--people were still wearing "dress turtlenecks" when GF was filmed.