I have these yellow green patches all over the back and sides of my tank. They are very hard to SCRAPE off. I need to use nail finger nail to scrape them off. A magnetic scraper wont cut it, just like coralline. The tank in six mos. old and little coralline on tank itself. Since I started getting this, there has been some definite purple coralline coming in around the same area. Is this just immature coralline growth or something bad.