Hi.. Thanks for the query.. You have a white coating and Avery mouth too that is a clear indication of yeast Infection or oral thrush.. As during the day you scrape it off so it disappears but at night time it gradually accumulates again.. You should start gargling with Nystatin mouthwash and suck Clotrimazole lozenges.. To enforce nasal breathing you can stick a vertical strip of microspore tape on your closed lips before sleeping at night.. But do it only of there is no obstruction breathing through nose. If there is still dryness you can start sticking xylitol patches in mouth over the inner cheeks.. Sip on water whenever you get up at night.. If problem does not resolve get an examination done by a Dentist.. Hope this helps.. Regards..