Gently blot the stained area with a clean towel as soon as possible before it dries.
You can make your own gentle silk stain remover out of a couple tablespoons of either white vinegar or lemon juice, mixed with equal parts lukewarm water.
Test the solution on a small, inconspicuous area of the silk before dabbing the stain with it using a clean cloth. Do not rub the stain.
Then wash your silk pillowcase as usual, inside your Pure Silk wash bag on delicate with cold water. Hang or lay flat to air dry.