Most algae are actually very dark green and in low light situations appear to be black. Algae will colonize any place in the yard where there is moderate to deep shade, the soil is more or less continuously moist, which may be due to poor drainage, the temperatures are continuously cool, and the soil is poor (lacking in nutrients). They can be harmful if they grow so luxuriantly that they smother grass or other small plants. They also tend to seal off the soil and make water penetration more difficult. Finally, they are slimy and slippery and may cause a safety problem in your yard, especially if they are growing on a shaded sidewalk or in a lawn area.
Usually the main causal factor in the presence of algae is excessive moisture and the more or less continuous presence of moisture. I have found that many sprinkler systems are installed so that sections of the yard are irrigated. Unfortunately, the stations often include both areas of sun and shade. If the station is set to provide adequate water to the sunny area (the usual scenario), the shady areas get continuously overwatered. The result is often algal growth. Drying out the wet area is an important step in eliminating the algae problem. Sprinkler heads are available, which can be adjusted to deliver different amounts of water, so that the variable water needs within a sprinkler station area can be accommodated. Improve the drainage in wet areas of the yard by changing the contour so wet areas drain. Incorporating organic matter into wet flower beds can also improve drainage. Sometimes judicious pruning or thinning of plants can improve air circulation and light penetration so that areas dry out between irrigations. Improving soil fertility can help to improve the vigor of plants so that algae are placed at an disadvantage. Finally, copper and certain iron-containing fungicides can be used to kill algae. Copper sulfate can also be toxic to higher plants. Therefore, read the label and follow directions exactly for dosage and treatment intervals.