Attach a high-powered nozzle to your garden hose.
Turn the hose on, and point it at the algae on the wrought-iron furniture. The high-powered water will blast away most of the algae.
Mix 1 gallon of water with 1 tbsp. garden detergent.
Dip a soft-bristled brush into the mixture, and brush over the surface of the wrought-iron furniture to remove the rest of the algae.
Rinse the furniture with clear water.
Let the furniture dry completely.