Stains occur where people lean back on the wall with their bare skin. Body oils and sweat then stain the walls.
A light sanding may get rid of existing stain on walls and benches.
To remedy the recurrence, simply install backrests on the upper bench walls. When sanding the backrests doesn't resolve it anymore, replace just the backrest.
Click here to learn more about backrests:Two Board BackrestsThree Board DeluxeDesigner