Green algae is the first sign you have lost control of your water environment. There are many parameters involved in 'why', but I can help you spot what is dominant in your unique mix:
 
Exposure to sun
Poor water flow in parts of the pool
Insufficient conditioner
High alkalinity water
Substandard Chlorination process
 
The longer you allow algae to persist, it will really embed itself inside:
Steps & hand-rails
Skimmer mouths
Robotic pool cleaners
Returns
The filter itself
 
Have your case inspected, chemistry checked and only pay for reasonable analysis & correction time.