coralline doesn't grow that fast. it's probably cyano
. see if you can wipe it off with your finger. also, coralline algae and anything else that's alive on the live rock you bought will live with just ambient lighting from the window or house lights. i have more coralline algae in my sump which is lit by curly-Q energy saver bulbs than in the display lit by halides.
unless you have corals in the tank, i would turn the lights on for a few hours a day, so you can enjoy the tank.