Shock your pool before the storm hits.

The green is algae, and the storm hands it everything it needs: rain dilutes chlorine, debris adds nutrients, and a power outage stops your pump. So hit it first. Add a heavy dose of chlorine shock and run the pump for a few hours before the rain starts. If you can, toss in a tablet floater for insurance. After the storm, test the water and shock again if needed. That is it. No magic.

The people who get green pools are the ones who wait until after. Do not wait.