I use copper tongue scrapers to get rid of the excess gunk. It’s taken serval months with intermittent fasting to get my tounge relatively clean but I’d imagine the process would quicken during prolonged dry fasts.
It’s yeast. It means you eat too much sugar/carbs. If you still have white tongue and you’re Keto, there’s still food the yeast is living off of, or you’re not strict enough and have considerable insulin resistance. I notice being strict carnivore gets rid of it, and if I’m Keto, eating plain Fage 2% Greek yogurt helps me eliminate it. The bacteria in the yogurt compete with and beat out the yeast.