Moloch! Moloch! Nightmare of Moloch! Moloch the loveless! Mental Moloch! Moloch the heavy judger of men!
Moloch is an ancient Canaanite god associated with sacrifice (especially child sacrifice), sometimes by fire. The speaker could be saying that the "best minds" of his generation have been sacrificed to society.
The Harlem Renaissance/New Negro Movement can be seen as an explosion of artistic expression. Perhaps Hughes is implying that the unrealized dreams of black people in America have exploded outward in this artistic movement.