There are many reasons for the white coating on tongue.
Poor oral hygiene which leads the tongue papillae to swell and become white.
Tobacco
Alcohol
Dry mouth
Dehydration
Certain medications
Leukoplakia
Oral lichen planus and the list goes on.
So it's better if you consult a dentist.
Health Tips
Try and maintain good oral hygiene i.e. brushing twice, flossing, cleaning the tongue.