Good question! No, this algae is from under-chlorination for your CYA level. Or, in other words, you fell below the target range, and probably below the minimum on this chart [FC/CYA][/FC/CYA].
If you really do the
SLAM Process (keep FC at your Shock/Slam level until the pool is bottled water clear - your phosphates could be 99999999 and it wouldn't matter because you would have ZERO algae.
So again, great question, but phosphates are just a profit machine for pool stores robbing their customers blind.
- - - Updated - - -
Just to make sure you understand what
SLAM Process is saying: Get FC to 16, keep pump running 24/7 if possible, brush at least once a day if possible, and test FC 2-5 times a day (and bring back up to 16)
This will be a few days - or potentially longer. Does your sand filter have a PSI gauge? If so, write down what it says after your next backwash. Then don't backwash again until it is 25-30% higher than that. Backwashing too often actually slows down clearing it up.
Also, you're off to a great start with real test kit, brushing, vac, backwashing - I'm really glad you found us!!!