I just saw someone post about this same issue. It may have been over on a Facebook group. If I remember correctly they used soapy water and a toothbrush and it came off but Im not 100% positive. If I can find the post I will get back with you.
In the past I've made an excellent stain-busting solution by added 1/2 a tablespoon of both Dawn dish soap and white vinegar to 1 cup of hot water. Stir together and, using a soft-bristled toothbrush, tap off the excess before gently apply to the stained cloth in small circular motions. Use a soft towel to dry the area a bit before re-applying as needed. Once you're satisfied with the results allow the zipper to completely dry - about 12-24 hours - before using the item again.