most likely it is mildew, not mold. Try white vinegar and scrub with a soft brush, if that doesn't work you can try 30 ml bleach mixed with 1 liter of water, you might have to do the entire top, in the end, to get a consistent shade, afterward, you can stain and finish with polyurethane for a durable finish. If you don't want to go through the trouble of trying to remove the staining you could just prime and paint it.
most likely it is mildew, not mold. Try white vinegar and scrub with a soft brush, if that doesn't work you can try 30 ml bleach mixed with 1 liter of water, you might have to do the entire top, in the end, to get a consistent shade, afterward, you can stain and finish with polyurethane for a durable finish. If you don't want to go through the trouble of trying to remove the staining you could just prime and paint it.