Halimeda: The Cactus Algae by Kirby Adams - Reefkeeping.com
On the rocks that have Halimeda but no other corals, you can pull them out of your tank and scrub off the holdfasts with a toothbrush. On the rocks that have corals, you can still do this but it's a good idea to have a squirt bottle filled with saltwater to periodically spritz the corals with.