Arrived home today with some weird dark stains in our feets. Has the oil spill reached Koh Lipe?

I'm in Koh Lipe right now. After spending the day on the sunset beach, when we arrived home, me and my girlfriend noticed we had a coupe of large black stains in our feet. It was very hard to get them off. Even after washing it with soap and water for a very long time there's still some light residue left on our skin. Anyone knows what this could be? Can this be oil or tare residue from the oil spill?