This would fit best on the actual product page. There you can find a place to comment. You could also get information from the actual reviews- Fleshlight Girls Stoya Reviews. If you have more questions after reading the reviews, you could message some of the contributors who have done reviews on this item.
Cornstarch has never left any kind of stain on any of my toys, Fleshlights included. My guess would be some form of mold/mildew from not being dry completely before storage. Try some Isopropyl alcohol and see if that gets rid of it.