[jimcol]

I have the same issue and agree with Dick and Mike. Tried multiple things to clean it. Nothing works well. I'm really not even sure it is mold but more discoloration from the water running across an already dirty surface plus the 18 wheeler diesel soot you pick up running down the highway. A water surface will pick up more than a dry surface. Not pleasant to look at but harmless. No one will see it from the ground.