Identification

• Some homeowners mistake moss or slime molds for algae. Algae grow in green, scumlike filaments that dry into a thick, black crust. Algae typically grow in wet soil with high fertility and sunlight. Moss grows in a green, threadlike mat and in wet, shady areas. Slime molds, a type of fungus, appear creamy-white or gray and grow in streaks or patches, which then dry into a crusty gray mat.
  
  

Risks

• Algae are not parasitic plants, but they form a thick, impermeable layer sealing the soil. This kills grass by making it impossible for turf to get water. Wet algae are slippery, causing a safety hazard, especially on sloped lawns or walkways.
  
  

Removal

• Kill algae on your lawn with diluted copper sulfate or potassium salts of fatty acids. Rake dead algae off your lawn. Applying limestone inactivates the copper sulfate before reseeding the grass. Lawn care professionals also eradicate algae with mancozeb or other fungicides. After removing algae, address the conditions in your lawn that encouraged algal growth.
  
  

Preventing Regrowth

• Algae and mosses reproduce by wind-blown spores, so your lawn could develop algae again at any time. Good cultural practices reduce the chances of algae and other problems. Improving the lawn's drainage avoids overly wet grass. A core aerator reduces soil compaction and improves air circulation. Don't water too much, and have your soil tested to check whether you need to fertilize or adjust the pH of your grass. If changing the conditions of the area is too difficult, consider covering the area with mulch or another groundcover.