Marx deemed capitalism exploitative and saw a need for change, but he did not call the system unjust. Determining his thoughts on the matter largely depend on whether you look to his works as a young man largely founded upon liberalism, or to his later works.
–> Early Marx: critiqued capitalism as game with rules. Though the game is itself is exploitative, actions which abide by those rules are by definition, “just”.
–> Later Marx rejects the idea of justice. He believes that philosophy is a result of the economic system in place, and justice is a part of this superstructure (philosophy) created by capitalism. In communism, there is no need for a concept of justice or injustice, since it is a perfect material practice without contradictions.