Can you explain what Beneatha means when she says that "there are two things we, as a people, have got to overcome, one is the Ku Klux Klan—and the other is Mrs. Johnson"?

Beneatha is referring to what Ms. Johnson is symbolic of, which is the attitude of some black folks to resent people of their race who attempt to better their class position. It is very clear that she—Ms. Johnson—is one who holds this opinion. She comes to the Younger house with a newspaper that expresses the condemnatory views of the people in Clybourne Park towards black families choosing to move in to a white neighborhood. The article explains how a black family in a white neighborhood was bombed out of their house. She leaves the newspaper on the table when she leaves the Younger's house as a warning.
Beneatha realizes that not only do the Youngers have to worry about the racist white people who want to keep black folks from being upwardly mobile, but they also have to worry about other people of their culture who believe that they shouldn't try to raise their status.