During the night, most people sleep with their mouths closed. This environment allows bacteria that like lower levels of oxygen to thrive. In the morning, the white slime is left.
Could be from something you are using or consuming in the evening. Or possibly a yeast infection. Try eliminating things from your routine to determine if that is a factor.
It is possible that you have some type of a yeast infection in your mouth. This could be caused by many different reasons. You should consult your doctor for treatment.
Morning mouth is what many people call the white paste found in the mouth. It is caused by bacteria. Brushing well at night, morning and throughout the day can help.