Boasting waterfalls, rocks and natural features, some pools are more decorative than others. Unfortunately, some of these items get occasionally ignored during pool cleanings, causing algae to build up from water splashing or running over them. Once you discover a coating of the green slimy stuff, you'll need to clean it off so it doesn't contaminate your pool.
Remove the rocks from the pool area, if possible. It is better to clean them away from the pool water to avoid contamination.
Place the rocks on a plastic disposable sheet and pressure-wash them to remove most of the gunk. Turn the rocks over and repeat on the other side. Rocks can withstand more pressure that things such as wood, so turn up the dial on the machine to increase the pressure.
Pour straight vinegar over the rocks one by one and scrub with a brush. Keep a watering hose handy to rinse off the vinegar.
Leave the rocks out in the air and sun for a day or two to kill any remaining algae. Turn the rocks over halfway through so both sides can dry.
Return the rocks to their places in the pool area.