In the morning when I wake up my mouth feels like sandpaper and all slushy. Why?

Mouth breather. If you are mouth breather at night either knowingly or unknowingly , your mouth will be dry . This happen at times when you have some kind f problem with your nostrils . You might have to use nasal decongestant .
Mouth issues. Without more history it's impossible to think of anything in particular that does that. Acid reflux can cause acid penetration in the mouth and unpleasant sensation/taste. But that's the only thing i can think of without more history.