I believe that power washing is not recommended for treated softwood. Power washer is strong enough to abrade the wood so leaving more ridges and furrows to hold more water and increase algae and moss growth.
Oxalic acid timber cleaner should remove the green. Vinegar may also do a similar job. There are various products to remove mold from timber try searching for local products.
Green algae/mold will always come back when there's moisture as on bonsai bench. Painting the timber with a good decking paint/stain/oil will slow it down but all need to be reapplied every 6-12 months to be effective.
The alternative is to learn to live with green benches.