I think you have two separate things going on there.
Off the decking you have an algae build up, which should pressure wash off, it may have marked the surface.
The second issue, I think is a metallic run off, either from bolts or framework, this is harder to remove, pressure wash, it may lighten it, it may remove it completely.
Maybe match up the paint colour and go in with the offer to repaint specific areas, if it doesn't clean off properly!!
Rob