Antibiotics can kill beneficial bacteria and allow a fungal overgrowth called thrush that looks like white patches on the tongue and sometimes also the inside of the cheeks and/or the roof of the mouth. Thrush can also be caused by a compromised immune system or inhaled corticosteroids.
Otherwise it is most likely poor oral hygiene. My dentist told me that scraping your tongue is almost as important as brushing and flossing. If you let food debris and bacteria coat your tongue they can cause bad breath and cavities even if your teeth are clean.
You can use your toothbrush to clean your tongue but purchasing a tongue scrapper is a good investment.
Multiple possibilities. Might be a reaction to something you ate. Might be just a coat of dead skin cells and some lactose bacteria or similar harmless beasts. Might me a mycosis. Might be a symptom of an illness.
Are you sick? A cold? Sore throat? Then it might be a symptom.
Can you remove the white stuff by scratching it off, for example with the side of a small spoon or with a toothbrush? Then it's just a coating and most probably harmless.
Do you have a strange/disgusting taste in your mouth that doesn't go away by teethbrushing? Does it have a bad scent? Then you should go to a doctor.
Its called it could be thrush from to much yeast in the body. Im not exactly sure what you do for adults but if its like that for a few days see a doc so they can give you something.