u/Pure_Ad_6027

Customer would like their house soft washed. My question is whether or not this hardie board is oxidized. The shiny section in the photo is where I rubbed my hand over it. It also left a dark residue on my fingers. I told the customer there may be oxidation and that a soft washed will not fix this. Should I walk away? What problems could happen if I do a soft wash?

Im using sodium metasilicate and then an oxalic acid but this green algae does not want to come out. Rinsing with low pressure to not damage the wood. What am I Doing wrong? Any help is appreciated