Cyano needs phophates to live, it does not need nitrate. Atleast not in the form you are testing for. Your phospates may test zero, but only because its being absorbed as fast as it is released. Don't scrub the rocks. You need to remove the cyano from the tank, or it just becomes fuel for the next round.
I tryed everything to get rid of it. I used airline, and rigid tubing to suck it off the sand and rocks. I cut my light cycle, increased the time I run my phophate reactor, increased my temp and PH. It always came back, day after day.
Justin sold me "Red Slime Remover". You have to turn off your skimmer for 5 days. But it never came back. In 5 days I fired the skimmer back up, and everything has been great.
I never liked the idea of useing chemicals, but this stuff works.