I've seen that on speakers before and wondered what it's from. I've even owned speakers that had that kind of look on woofer surrounds.
Is it possible that in production, somebody along the line polishes the cabinet by hand, with a rag/towel, and some got on the woofer surround?
Get a soft sponge, dampen it, and *gently* wipe the rubber surround. See if it cleans up. I was able to clean mine just fine (not Polks).