Thick White Mucus. Main causes of thick, non-translucent, white mucus: 1  Drinking milk or other thick drinks can make originally clear mucus thick. 2  Milk may cover the back of the throat, but milk itself does not cause mucus.3  In gastroesophageal reflux disease (GERD) , gastric acid may back up from esophagus into the windpipe and throat.hick White Mucus. Main causes of thick, non-translucent, white mucus: 1  Drinking milk or other thick drinks can make originally clear mucus thick. 2  Milk may cover the back of the throat, but milk itself does not cause mucus.