A very simple solution for your problem is the Mucus-free diet.
Try to live for one week on the Mucus free diet and see if the white line forms.
The Mucus-free diet is the diet free from 5 major group of foods: milk, sugar, grains and seeds, fried foods, industrially processed foods which usually contain at least one of the 4 foods mentioned befor.
You can eat meat, fish, vegetable, fruit, eggs, small amount of nuts, olive oil, coconut oil, etc .... but you avoid the 5 groups of foods mentioned above.
The results of your diet may show already after a few days, you may not need 7 days to get the results.
What you describe is a sign of intolerance to grains or milk or sugar.