Honestly, MLK Jr seemed a lot less communistic than other civil rights figures of the time. MLK Jr preached about Jesus and once said “Communism and Christianity are fundamentally incompatible”.

King was constantly labelled such because at the time the USSR and the Cold War were still the largest threat to American society and way of life and perhaps even continued life on earth, meaning it was used as a label for your enemies in the sociopolitical sphere. It didn't help that the Russians claimed to promote equality which was a word King used often.

Now Malcom X and the Black Panthers? Those were some commie mfers. Malcom X wrote a letter directly to President Truman claiming to be such, and the BPP hardly tried to hide it either.

The Sword and the Shield by Christopher Andrew and KGB defector Vasili Mitrokhin:

King was probably the only prominent American to be the target of active measures by both the FBI and the KGB. By the mid-1960s the claims by the [Communist Party of the USA] leadership that secret Party members within King’s entourage would be able to “guide” his policies had proved to be hollow. To the Centre’s dismay, King repeatedly linked the aims of the civil rights movement not to the alleged worldwide struggle against American imperialism but to the fulfillment of the American dream and “the magnificent words of the Constitution and the Declaration of Independence.”

[...]

Having given up hope of influencing King, the Centre aimed instead at replacing him with a more radical and malleable leader. In August 1967 the Centre approved an operational plan by the deputy head of Service A, Yuri Modin, former controller of the Magnificent Five, to discredit King and his chief lieutenants by placing articles in the African press, which could then be reprinted in American newspapers, portraying King as an “Uncle Tom” who was secretly receiving government subsidies to tame the civil rights movement and prevent it threatening the Johnson administration. While leading freedom marches under the admiring glare of worldwide television, King was allegedly in close touch with the President.