Our house has Hardie board siding. It is getting covered with some kind of greenish stuff, possibly mold or moss, that does not easily wipe off. (This is New Zealand north island which has a rainy, damp winter.) Can I use sudsy ammonia cleaner to clean it? I tried this cleaner called Handy Andy, which is like what I know as sudsy ammonia, on a small area and it seemed to work. I used a soft brush to brush it on and scrub.
If I continue to do this will it harm the Hardy board? What is the best way to clean Hardie siding? The board has a color (creamy whitish pink or pinkish white), but I cannot tell whether it is painted or was colored to begin with. I think the latter as I can't see the smallest sign of paint chips, drips, or splashes anywhere.

Hardie siding unpainted is gray so I'd think you have been painted. I would use bleach water 1:4 or a commercila mildewcide and then power washer at 2600psi.