Hi.. Thanks for the query.. Greenish coating on the tongue can be due to the infection caused by Chromogenic bacteria causing discolouration of tongue.. Many a times coating on tongue can begin with white and can change colour due to action of certain agents like mouthwashes, bacteria, certain medications, foods etc.. A condition known as Hairy tongue can also be a cause.. So my suggestion is to consult an Oral Physician and get evaluated and a microscopy of scraping from tongue and tongue swab culture can be done to rule out the exact cause.. For now you can gargle with hydrogen peroxide solution in water.. Use a tongue scraper to clean your tongue.. Eat more of hard foods like apples, carrots etc.. Hope this helps.. Regards. Dr. Honey Arora, Dentist.