Yes I read that they'd eat coralline but I'm okay with it. It's so tiny and the coralline grows so quickly. I don't think I can get ahead of it.
I also read that pro-urchin people believe the rasping of the coralline actually helps in distributing the spores and encourages coralline growth in the tank. Interesting idea, isn't it?
As long as it's not hurting corals or clams I am cool with it.