Many people who have lr and algae problems do whats called "cooking" the rock. Take the rock out of the tank and put it in a brute or rubbermaid container with fresh sw, a ph and heater if necessary. Keep the rock dark. Change about 50% of the water every week. In about 4-6 weeks, the algae will be dead and gone and the phosphate that may have accumulated in the rock with be purged.