I keep waking up with a thick white layer on my tongue that i was told is not thrush. what could it be then?

Dry skin: The surface of the tongue is covered with an adaptive surface of rough tissue that ages and sheds over time. It can dehydrate if you're a mouth breather while sleeping.It can also trap milk solids and other food debris.It tends to turn a pale grayish white & shed periodically, pushed up by pinker younger tissue.

Dry mouth: If you are a mouth breather, that can contribute to a thickening of the saliva while you sleep. The surface of your tongue is covered with tiny "whisps" of skin that grow over time and contribute to the white appearance. It's always recommended to use a tongue scraper or to clean it with your toothbrush to avoid problems.

White tongue: The tongue is known to have many variations with pigmenting, size, appearance, & not always due to disease. Causes of a tongue that looks white include: lichen planus (responds to steroids), white sponge nevus (familial, benign, permanent), smoker's leukoplakia (2% risk of cancer, needs biopsy), hairy leukoplakia (due to epsteinbarr virus), warts (due to papillomavirus), "thrush" (rx antifungals).