In reply to andyclimber:
Sadly, that - along with other stuff on the Eagle Stone in the same yellow paint - got done last summer. I contacted the PDNPA to see if they had a good chemical that was also environmentally acceptable, but they had run out. They were friendly and helpful - one of their staff went and did a lot of wire brushing to improve it on the Eagle Stone until it looked more like a lichen stain...ish. I felt that wire brushing was also damaging, but I don't think there is any one good solution. I have also given it a bit of brush/scrub sometimes when i ride by.... it's gradually going.
Now you have reminded me, I'll spend some time scrubbing "Leon" this week.. I'm pretty sure that an effective chemical would leave a clean stain that looks just like "leon"!