Vitamin C is for metals. Organic stains need chlorine. Setting a puck will likely bleach it, but pucks are acidic and might etch the plaster. A better bet would be just keep your FC at the high end and be patient. Make sure you get plenty of water circulating around the steps, even if that means brushing more. And when you add bleach, add it near the steps and then brush good so they get a good jolt before it mixes.