Microscopic plant life, or algae spores, are constantly introduced into swimming pools and spas via wind, water, swimmers, etc.
Without correct sanitizer levels, circulation and maintenance programs these spores rapidly bloom.
Even in the mildest cases algae infestation requires expensive chemical treatment and daily attention.
When treating an algae infestation you must determine why it occurred. If not resolved, infestation will become a regular occurrence.
For an algae spore to bloom it must attach itself to a surface free from excessive sanitizer and water flow, and preferably in an environment of high pH. Prime spots are transitions between the wall and floor, steps and cracks in the plaster.
Once established, the spores produce a waxy covering, preventing penetration of sanitizer. With neglect, the spores rapidly reproduce, discoloring the water and forming a slippery layer on all surfaces.
Treat all algae problems quickly. The longer they are left, the more costly it is to rectify the situation.