Hey Reefer1 - Welcome to Reef Sanctuary!!
I think getting a phosphate test kit would be the next step. When was the last time the RO filters were changed?
We know what fuels algae (P, N, Light)....so, lets try and determine where that fuel is coming from. If your water is testing positive for phosphates, then we need to figure out where they are coming from. There has been a lot of discussion lately on phosphates and algae issues, so I've taken in a lot of information. Here is what I've learned. Phosban works to get the phosphates out of the water column...not where it is originating from, so key is to figure out the source. We can all point fingers at substrate, food, etc...but unless you test, you can't be for sure, right? The following tests are ones that mojoreef recommends in determining where the phosphates are originating. Test the phosphates in your sand bed (if you have one) by taking a water sample from 1/2 inch down in the sand. Do another in the middle of where the algae is growing on the live rock. Another test from the RO/DI you are using for your top off water. One way we introduce phosphates is by food, so just to let those that don't know - if you soak your food in RO/DI for at least an hour, it will pull off the phosphates. Then you strain and discard the water - not feeding it back to the tank. Simply rinsing in RO/DI doesn't do enough. Also, do you know what the phosphates are in your mix water?
Another thought...what is your photoperiod, and how old are the bulbs?