Stains from direct contact with hair/skin are not uncommon; it happens to sleeping bags, jackets, etc. as well, over time. You can try cleaning the spot with lukewarm water and some mild detergent (the one you use for wool, for instance), but we can't guarantee it'll completely disappear. Luckily, the pad will work just fine, even with some stains on it. Make sure you let the pad dry completely before packing and storing it.