How do I clear up a pool that has turned green from algae after rain?

Shock the pool with a double dose of chlorine, run the filter 24/7, and brush every surface. Rainwater dilutes chlorine and brings in phosphates, so use calcium hypochlorite or liquid chlorine at 2x the normal dose with pH between 7.2 and 7.4. Brush walls, floor, and steps, and run the pump continuously until clear. Backwash or clean the filter when pressure rises, and skip algaecide or flocculants until the water is partly clear. It may take two to three days.