Urchins and algae
I have a sea urchin which is about the same size as yours, but I've never seen it eating any nuisance algae over the last couple of years (maybe our urchins are closely related!). I haven't seen it on the glass in my tank, although I do have a large amount of rock in my set up ... about 230 pounds in a 90 gallon tank. Maybe the urchin gets its kicks just eating coraline off of rock?
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