Quite a few guys won’t wash hardi board due to possibly fading or streaking of the paint.
If you were able to wipe it off with your finger I would guess that the previous washer let the solution dry before rinsing. If so you should be able to apply your regular house wash mix, let dwell, and rinse. I would use a weaker than usual mix.
If that doesn’t work there’s always brushing it but I personally wouldn’t do that unless I knew it was going to lead to a ton of future work. I would also charge a pretty penny to do so. You likely won’t be able to brush only certain areas but rather the whole house or at least the whole side. It just depends on how bad the paint is oxidized. Do test areas of both the regular house wash mix and if you decide to brush. Don’t just start going to town.
I’d definitely get something in writing or sell it as a paint prep. If it ends up not needing painted great. You just have to let them know the risks involved. I wouldn’t risk it for just any regular old customer that’s for sure. Jobs like this can end up biting you real fast so proceed with extreme caution.