Answer: He disagreed with its separatist views.
Malcolm X was a civil rights leader and black activist in the 1960s and 70s.
When twenty-one, Malcolm X was arrested, and during his time in jail, he discovered the writings of Elijah Muhammad. He was the leader of the "Nation of Islam," also known as the "Black Muslims." This was a separatist group that argued that whites and blacks could never reconcile, and that white people were the origin of the suffering of black people.
Upon release, Malcolm joined the Nation of Islam and became an active civil rights leader. However, he grew increasingly disappointed with the group's lack of activism and separatist views. He eventually left the group, although he remained a Muslim.