[BlueAbyss]

I would think passing the algae through a digestive system (the urchin's) would kill or denature the algae... That being said, I'm sure that an urchin (depending on the size of the tank) can eat coralline pretty fast. I'm sure you'd have bare spots on your rock... but as far as the coralline on the glass, I think you may be stuck. I've never heard of a snail eating coralline. Or an urchin climbing glass...