06 August 2021
Red algae (also known as red slime algae) isn't algae, it's a bacterial growth. It presents as you described. Another sign is discoloration of colors around stairs, in corners and around return jets and skimmers.
It has a variety of causes but most often it's caused by a lack of maintenance and out of balance pool water.
Because this is bacteria it's gonna take more work to get rid of it.
Remove all pool toys, ladders, steps and anything else that can be removed from the pool and thoroughly scrub them with. 50/50 solution of bleach and water. Don't forget any floats that aren't in the water.
Thoroughly wash all swimsuits
Clean your filter. If it's a sand filter, thoroughly backwash it. If it's DE filter, backwash and use fresh DE to recharge it. For a cartridge filters, take the cartridge out and thoroughly clean it. Replace it if it's old or has surpassed its cleaning life.
Test and balance the water. Since you didn't provide any information about the type of pool, size of pool, volume of water, the type of filter or any water test numbers, we don't have any information to go on there.
Brush the pool. Red algae clings to surfaces, especially slick surfaces. Can't over stress this....brush the pool at least twice a day.
SLAM the pool. Slamming the pool means bringing the chlorine to shock levels and maintaining those levels until the algae is dead. Again, since we don't know what the chemical readings of your water are (especially the CYA level) we can't be of much help there.
The filter should run 24/7 until the process is complete. And it should be cleaned frequently.
Let us know if you have any questions.