This… is yellow algae
This… is green algae
This… is black algae
The differences are subtle.
Yellow algae starts to form on the walls of your pool and can be easily brushed away with your hand. It has the color of a blade of grass.
Solution: Raise pH to >7.8 and add Yellow Out and shock. Or use Algae Solution and shock.
Green algae infects the water very fast and is a forest green color.
Solution: Drop the pH to 6.8 and raise the chlorine level to >30ppm.
Black algae is tricky. Sometimes it looks black but it can look yellow or green. Black algae doesn’t infect the water and doesn’t brush off easily.
Solution: Silvertrene and a stainless steel brush if it’s widespread. If it’s just in a few spots you can grind a chlorine tablet into it.