Make sure your doing small frequent water changes. At least fifteen percent at least once a week. A good salt mix will replenish the needed levels to grow Coraline.
When doing tank maintenance, and providing you have some rock with a little Coraline on it, scratch at it with your thumb nail here and there to spread the spores.
I have also had an lfs
tell me to add a small urchin to the tank. They will feed on Coraline but in the process break lose and spread spores and actually contribute to its spread. I do not however know how sound this advice is and would think that in the absence or easier alga's for the urchin to eat your Coraline could disappear pretty quickly.