Algae will grow in swimming pools, fish ponds etc when there is not a proper amount of chemical to keep it from forming, it is just natural.
Alge and bacteria also the storms may blow things in pool and leave may turn pool green
The same reason mold gets in your shower. Its spores float in the air/hitch rides to get to water sources.
They are jealous of other pools.
Like as if i was getin in a pool it would be green slimey and gross!!!
