Some forms of algae can be very chlorine resistant. The easy solution is simply to raise chlorine levels to compensate. Do this:

1. Add 3 gallons of PLAIN 6% chlorine bleach in the late evening. Brush the algae AFTER adding the chlorine.
2. Test the following PM; if the chlorine is ABOVE 15 ppm, skip the dose. Otherwise, repeat add 2 gallons of bleach.
3. Brush every other evening, and continue with the 2 gallons, if under 15 ppm, EVERY evening, for 4 days after NO trace of algae and NO trace of green is left.
4. Do NOT turn the pump off during this period; clean the algae that won't filter AFTER 110% of the algae is LONG dead.