Can't remove dark footprints from metal roof

I am sorry ... I do not have any ideas on this. Hopefully someone else will.

Hi! In early May we had our fiber cement sided home powerwashed (with a bleach solution). The men had to step onto our front porch's black metal standing seam roof to be able to scrub the second story aluminum gutters. The cleaning was done around 9 am and the roof sits on the west side of our home, so the metal should not have been too hot and I don't think the temperature of the metal was an issue. After they left and all of the water dried up, I noticed black footprints all over our metal roof. The power washers came back and tried to clean them off with more solution (I'm not sure what they used) and a soft bristle brush. It did not work. The footprints are still there, but they have faded some over time. I, nor they, don't know what caused the prints to occur. They also don't know how to get them off. They've offered to paint our roof for free to get rid of the unsightly marks, but I don't want to add a future maintenance item on our list when the roof will have to be periodically painted. The roof is only 5 yrs old and looks great otherwise. Any ideas what could have caused the footprints? Also, any ideas on how to remove them? Any information you can share will be appreciated. Thanks!