If your calcium and alkaline is in range and there's enough light, it will grow. If you want to jump start it get a rock with some on it. Put it in a bucket of tank water and scrape off as much as you can. Turn off all pumps in tank so water is still, and sprinkle the scraped off coraline onto rocks. Leave pumps off for an hour or so to let settle.
If cal and alk is not in range, don't bother