Controller gunk can be caused by a variety of things, but the most common culprit is sweat. When you're sweating, your body releases a mix of water, salts, and oils. All of that gunk gets trapped in the crevices of your controller, and it's tough to get rid of. If you notice that your controller's buttons are sticking or that the controller feels sticky when you're done playing, it might be time to give it a good cleaning.