Conan, do you presently have any herbivores in the tank to help prevent this problem? If not you may consider some, however , to answer your question directly, you can scrub your live rock with a brush to remove the algae. I would suggest that you scrub it in a separate bucket in saltwater from the original tank. This will help prevent any loss of your desirable pink and purple algae. Now getting back to the herbivores. More most be known about the livestock in this aquarium to be able to make the correct recommendations for what algae eaters might work best in this situation. Yet another thing to keep in mind is, if this liverock has absorbed a large amount of phosphates from the what ever water it was in in the past it may be a long drawn out affair getting rid of it. DOH! Either way good luck. 8)