First of all, please note that neither of these men was alive at the time that Plessy v. Ferguson was decided. Therefore, they could not have had any impact on the case.
If you have to make a connection between either of these men and Plessy, you could say that King worked to try to overturn the court's decision from that case. King was working to promote racial integration and make segregation illegal in the United States. Racial integration would go directly against the Court's ruling in Plessy.
You could not really say that Malcolm X worked against Plessyso much because he was not working for integration. He was more in favor of black nationalism and black pride. He was therefore (until near the end of his life), not very interested in integration.