I have two purple urchins that I acquired with my liverock. It loves munching on my coraling, will not touch hair algae, and is a real nuisance with knocking over corals and munching on the leather corals I (tried) to keep in the tank. I can only vouch for the purples, as that is all I've owned. They are great grazers though, althought mine won't touch hair algae.
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