Rock is , well... just rock. I think you are referring to Coraline Algae, which is a crusty hard algae that grows on live rock. It's very pretty and much desired. Red, pink, blue, purple, yellow and green..oh and orange. The orange turns red ... the pink turns purplish blue... the yellow turns green. Most see the pink in splotches first...which later will become blue purplish, and the reds will turn almost velvet color and very dark ( I had the deep red)...all coralline is hard and crusty. There is one type that actually branches out in scales.
Keep your calcium parameter between 450 and 500 for optimum coralline growth. You do need to seed the coralline if there is none already in the tank on the rock....to do that, take a rock already covered with it, and in front of a power head, scrape off the algae with a hard brush (marked fish only) That will send spores out. Coraline likes plastic better then anything else, so the power heads and intake/output tubes will be covered first. The good thing about that is the coralline will cause the plastic stuff to blend in with the décor, and not look so obvious.