get a rock thats pretty well covered in coraline, and scape it with the tooth brush like Sweet said. I didnt have any in my tank for like 6-8 months, then did this...now i have coraline EVERYWHERE; on the rocks, the glass, everywhere...I have to clean the glass every week or so, so the coraline doesnt build up.
All the conditions of my tank were the same before and after i scrape the algea covered rock....