There is no way to get rid of the stain without causing more damage. What happens when you add various cleaning chemicals to a book-cloth cover is that they go in but they don't come out. Also, any moisture has the potential to create its own stain by moving particles away from the center of the moisture to create a ring. Although I don't think this staining on the cover falls necessarily under the term foxing, the answer for any kind of foxing is to learn to love it. It is a natural part of the aging process for books that were made with certain materials and kept in certain environments. The exception is when the foxing is on a document that you can wash thoroughly in order to remove whatever you put in to clean it. This is done in conservation lab where the chemistry is carefully controlled.
I do have several courses where I talk about cleaning book-cloth and adjusting the color. In this instance I think just dry cleaning methods with a less is more attitude will serve you best.