If you have an outdoor rug that’s been covered in algae, there are a few things you can do to clean it. First, use a garden hose to wet the rug and then turn on the water full blast. This will help knock the algae off the surface of the rug. Once it’s wet, use a broom or a mop to scrub the algae off of the rug. Finally, rinse the rug with water and dry it off. ..