Good flow helps, since it spreads around. You can have a rock that's covered with it and a rock next to it that has none, and with a powerhead blowing across them (starting on the one with coraline) it'll spread to the other rock in no time. There are also some salts that are formulated for coral growth and they help with coraline as well, I've used Oceanic and Red Sea Coral Pro salts with good success in coraline growth.