Algae can vary in color with floating, green alga easily killed with the right pool chemicals. If you find you have yellow algae, these can be chlorine-resistant whereas black algae are actually bacteria. It is easy to ignore the first signs of algae build-up, and with a good old-fashioned scrub and vacuum, you can have your pool in great shape.
If there is considerable algae growth, you will need to add sanitizers and algicides. You will also need to test the pH and alkalinity in the pool, and balance the water. When the infection is too heavy, shock your pool, clean your pool’s filter, and run the pool’s pump for 8 hours straight.