Preserve black wood stain?

I have a job where they have a small 12 foot section of wood fence that is stained black. Only saw one side via pic and doesn’t look like much mold (will see the other side tomorrow).

Since it’s so dark any lightening will be noticeable, anything to avoid that, green ox, one restore ?

So far reading that one restore isn’t meant for this and green ox can stil lighten wood.

If no mold, just wash it with low pressure and some liquid laundry soap.I would wipe it with a wet cloth and see if any stain shows up on the cloth. Advise the customer of what to expect, ie some of the stain may come off.

Yea hoping that’s good enough. I will explain no way to get more aggressive chemicals on it without risky the stain.