You have to take into account the phosphates that the algae is using before it shows up on the test kit. I have the same problem but even with 0 PO4 readings I know there is phosphate in the tank.
Here are some steps you can take:
Thoroughly rinse frozen foods before feeding. The biggest source of phosphate is from the frozen food! Make sure all fish eat what you feed as soon as you put it in the tank (feed a little at a time). You may wish to keep your tank dark for 2-3 days straight. This will weaken the algae & make it easier to scrub off. Then try scrubbing the rocks with a toothbrush & do a major water change, as scrubbing in the tank will release spores.
I use Ocean Nutrition Flake Prime Reef food. I sure have algar problems as well. Does anyone use this?