Agree with flydan,
Can go either way depending on the quality and handling of the live rock.
Good things I've seen grow out on the rock I purchased have been some cool featherdusters, various types of macroalgae, spaghetti tubeworms, red, purple and green coralline algae, bivalve mollusks, and a funky sponge that I removed.
That was the good rock.
I bought some rock that looked pretty good at first, but nothing ever really "grew" out of it. I moved it down lower in the tank and stacked the good stuff on top.
Also bubble algae can grow out and spread pretty fast. Some folks don't care for it - some do.
Brian