only on new live rock

Getting this stupid green slime algae showing up on new live rock I added about 4 months ago. My original 45 lbs of Live rock doesn't have it. Water parameters look good... 0 phospate levels 0 amonia... Right now I have 2 10k 65 watt pcs and 2 actinic 65 watt pcs. both are about 3 months old...