Listed on script as episode #211, it's actually episode #212 - The Star System Syndrome. It's signed by writer Greg Weisman. The episode aired January 23, 1999.
It turns out that all those aliens we see in the movies are real aliens. When alien actors keep disappearing, Kay and Jay are sent to Hollywood to investigate. While solving the mystery, the MIB come face to face with every Hollywood stereotype imaginable, and some that are beyond imagination!