Believe it or not, saliva is very acidic and affects different fabrics differently. If you're seeing rust spots on your pillow, it's probably from where you drool a little in your sleep. It's not reacting to the pillow case because it's a different type of fabric.
You can use a spot cleaner on the outside of your pillow to remove the spots if they bother you.
I hope this helps!
Let me know if you still have questions.
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