I have cleaned a lot of outdoor benches, and the biggest mistake people make is reaching for bleach or a pressure washer right away. The finish on a street bench is usually more delicate than it looks, so the goal is to kill and lift the mold without stripping paint, dulling varnish, or roughing up sealed wood.
Start by removing loose mold and dirt with a dry soft brush or a rag. Do this outside so the spores do not spread indoors. Then mix warm water with a few drops of mild dish soap. Dip a soft-bristle brush or microfiber cloth into the soapy water, wring it out, and gently scrub the affected areas in small circles. Rinse with a garden hose on a soft fan setting, not a pressure washer. Pressure washing can peel paint, lift wood grain, and force water into joints where mold will grow again.
If mold is still visible, use white vinegar. Mix one part white vinegar with one part water in a spray bottle. Spray the area lightly, let it sit for 10 to 15 minutes, then scrub softly and rinse well. Vinegar is effective on surface mold and much less likely to dull clear coats or discolor finishes than chlorine bleach. For black staining that has soaked into wood, an oxygen bleach cleaner is a safer choice than chlorine bleach, but always follow the label and test first.
Always test any cleaner on a hidden spot, like the underside of a bench slat. Wait a few minutes, wipe it off, and check for color change, cloudiness, or softening. Painted metal, powder coating, spar varnish, polyurethane, and plastic all respond differently. If the finish turns milky or feels tacky, stop and switch back to plain soap and water.
After cleaning, rinse thoroughly and let the bench dry in the sun. To slow regrowth, keep leaves and debris off the bench, trim nearby plants, and consider applying a coat of exterior paste wax or a compatible wood sealant once the bench is fully dry. That extra barrier protects the finish and makes future mold cleanup much easier.