George's arrival changes the mood because he complains that Beneatha is giving them a lecture on their African past
Explanation:
George Murchison was apparently disgusted when he came into the house to find Beneatha dressed in an African attire and dancing happily with Walter to some African drums. He derided Beneatha for celebrating a culture which he referred to as "nothing but...some grass hut."
Beneatha accused him of being an assimilationist who was not proud of his heritage. So, George Murchison changed a happy scene to a tense one.
