The reason you have algae is because your tank has nutrients. The reason your tank has nutrients is because you added live rock. Live rock has a certain amount of die off and detritus that comes with it, and that die off and detritus has now decomposed and made it in to your water column as phosphates and nitrates. Your algae consumes these nitrates and phosphates to bring things back in line.
How do you get rid of the algae then? Well you have to get rid of its food source, the nitrates and phosphates that are in your tank. Some of them are floating in your water column still, and some of them are bound up in your algae. What you must do now is try to scrape and siphon some of your algae, and continue to do water changes with high quality salt and Ro/Di water. This will not be a one time battle, but rather a war of attrition. As you remove more and more of the phosphate and nitrates, you will have less and less algae.
Peace,
Jesse