i have seen many urchins eat coralline algae. my friend had a long spined urchin in his tank for a couple years and his rock always looked like he just put it in the tank- pure white. once he removed it, it turned purple over time.
i don't really see the big deal about coralline algae anyway. folks are always stating things like "coralline is the sign of a healthy tank", or coralline covered rocks won't allow any nuisance algae to grow on them", both of which are untrue. actually, it's just another component that uses up available calcium as far as i'm concerned. something i'd rather have available for my corals.