Honestly, the safest method I’ve used and recommended is warm water, a couple of drops of mild dish soap, and a soft-bristle brush or sponge. You don’t need harsh chemicals to lift most green algae. Wet the bench first, then gently scrub along the grain if it’s wood. Rinse thoroughly with clean water and let it dry fully in the sun.
For stubborn patches, mix one part white vinegar with one part water in a spray bottle. Spray the affected areas, wait five to ten minutes, then scrub lightly with a soft brush. Rinse well. Vinegar is mild enough that it usually won’t strip a sound finish, but always test it on a hidden spot first, especially if the bench is painted or has a delicate sealant.
Avoid pressure washers and bleach. A pressure washer can force water under the finish and cause peeling or rot, and bleach can discolor the wood and harm nearby plants. If you want extra protection after cleaning, apply a breathable wood sealer or furniture wax once the bench is completely dry. Keeping the bench out of deep shade and wiping it down periodically will also slow algae from coming back.