This is a sign of congestion likely acidity inside the body and sometimes candida. Scraping it with a spoon works for the top layers but it "grows" back, baking soda scrubbed with a toothbrush and left to sit for a minute or two works in cases of acididty as does the contents of a probiotic capsule or a mouthful of homemade kefir in the case of candida. Overall the problem root is congestion however and that must be addressed by colon cleansing and change of diet/lifestyle. My coating had reduced dramaticall since I have removed wheat and heavy starches from my diet during the day only eating starches at my last meal after veggies and protein. this is caused by incompatibilities for your own body