In order to get coralline growing strogly you first need a rock with some established on it first. If it's happy with your tank it will start to spread. A piece of LR
with some established coralline thickly growing on it purchased from a friend or LFS
is your best bet.
Coralline likes high calcium levels, and/or tanks with frequent water changes since the fersh salt mix will keep calcium levels elevated and stimulates it's growth. Coralline also loves actinic light, but will not grow as rapidly under white light. Conditions that are ideal for your hard corals will usually be ideal for coralline, but you really need a seeder rock to get things moving quickly.