The tongue can present strong visual indicators of a person's overall harmony or disharmony.l
You could be suffering from something as simple as dehydration to more complex such as thrush(yeast infection in mouth).
You also mention the tongue coating comes back after brushing it. Your tongue hasnt always had this coating. IĒd guess something is going on somewhere in your body and its showing up on the tongue. Iím not a doctor but.....
You mentioned seeing a dentist but not a doctor. Might be time to see one.
Here is a link to tongue diagnosis according to traditional chinese medicine. if your interested.