Sounds like thrush (oral candidiasis). I found that fungicides do not work, so I took drops of Lugol's solution (potassium iodide). That worked just fine. (A fungicide causes candida to mutate, so the problem recurs. Iodine just kills the fungus outright, and no side effects.)
Do you have allergies or acid reflux? Those could cause excess mucus. Something called NAC - N-acetyl cysteine helps thin out mucus and is worth a try, you would be able to find it in GNC or health food stores.
If you have sinus or allergy problems, using a nasal rinse before bed and following the recommendations for limiting allergies would go a long way to helping that in the morning. Let me know if you want more info on this.
REflux would improve by raising your bed up 6-8 inches at the top so gravity helps you overnight.
Also, make sure you are drinking enough water each day because dehydration developing overnight would also thicken up the mucus.
I think you have problems in your acid reflux. I believe that Tammy is right. You need to lessen up your acid intake and drink more water to prevent dehydration. A high fowler's position does not only prevent reflux, it also help the lungs to expand to its full capacity while you are sleeping.