Shock it and balance the chems.

Yes. Heavy rain dilutes your chlorine, dumps in organic junk, and throws off pH. Algae sees an open door. So: test your water, add a heavy dose of shock (calcium hypochlorite or liquid chlorine), brush the walls and floor, and run the pump 24/7 until it clears.

After the cloudiness drops, balance pH and alkalinity back to normal. You probably don’t need algaecide if you shock properly—extra chemicals just complicate things.

Don’t overthink it. A day of shock and filtration usually gets you back to swimming.