OK, you are on the right track. You want to raise and keep your FC at 20. You need to test and raise FC at least 3x/day.

Meanwhile you need to brush your pool every day to knock the algae loose, and you should vacuum the pool every day as well, preferably to waste, until it's clear.

I think what's happening is the algae is using up the chlorine and you aren't getting more chlorine in the pool fast enough to prevent the algae from growing back.

Carl