discolored area on metal roof

Hi, Our carpenter's tennis shoes left a dark area on the metal roof when he got on it to put siding on the shed dormer. What can we clean it with, it's not scuff marks but a dark area about 8 x 10 around it? It's only in that area so we are sure it is from his shoes because the guys that put the roof on did not leave these same marks. Thanks for any information you can give us. Dee Dee