Your water has turned green, the walls are slippery and covered with algae, as well as the bottom, around the skimmer, outlet nozzles and/or the steps?
Algae has developed in your water. There may also be phosphates present. Green water can also be due to poor water treatment: pH not properly adjusted, insufficient filtration time, not enough disinfectant (chlorine), over-stabilised water, etc.
What is the treatment for green water?