Serious help? Here you go...
Use formic acid ( Ask Carl for some
), remove it with 7/8 litres per min at about 1000 psi at 130/140c.
If you use your "pressure washer" you will saturate/erode the stone...if it then freezes it will remove layers from your "conservation" piece. Your heritage officer will love you then.
Tensids products work well but most do not come into the "eco-friendly" category ...most are labeled as corrosive, and require special handling. You will also need to make arrangements for safe disposal of your waste product. You will need a waste carriers certificate. Do you have a vacuum system for recovering the waste/contaminated water
Tensids graffi green might work depending on the porosity of the stone and depending of the type of staining...i think this is there only "safe" product.
The one my mate makes in his shed is label free, no dead fish ,falling down tree or hole in the hand sticker....and it works
How were you thinking of doing it?