I do not know if MLK was a commie or not.
Hoover thought he was, and did a good enough job convincing the Kennedy's of such that they agree to the surveillance of MLK.
For what any of that is worth....
Say what you want about the JFK and RFK, but they were staunch anti-communist, Bay of Pigs, October Missile Crisis, the ramp-up in Vietnam....
Hoover may not have had to try very hard to convince them.