Mister Charlie is a pejorative expression used within the African-American community to refer to an imperious white man, or occasionally, a black man who is arrogant and perceived as "acting white".
The expression was in use during the 19th century, much like the female equivalent, Miss Ann. Miss Ann was an expression used among slaves to refer to the woman of the house, usually the wife of the slave owner, and any other white woman that the slaves had to serve. Mister Charlie was the slave owner, or any other white man exploiting, or being condescending towards, slaves. The expression has fallen out of use by young African-Americans today.