They feed with five very sharp and very strong teeth plates that scrape
coralline algae and other organic material off the LR and other
surfaces. I had never heard the name before that I can remember, but we
learned at the MWRF that the scraping teeth unit is called Aristotles
Lantern.
So with that being their main food and their way of eating it, they have
trouble with anything else. There was an experiment done once where
Diadema Urchins were fed only Caulerpa. They died within a few months.
The same thing may happen if the only food is Nori, though I found a link where one hobbyist successfully feeds Organic Romaine Lettuce.
http://www.dailykos.com/story/2008/2/22/112152/903/835/461894
I wish you the best of luck, but if the Urchin at first seems to be
running all over the tank trying to find some food, I would suggest you
give it to someone that has the proper food. Especially if starts to
look lazy and doesn't move much, at that point it's almost too late to
put it in a tank with Coralline Algae, because then it may be too weak
to eat and it will soon die.
In the wild, most Urchins only travel a foot or two from their home,
eating the coralline and other algae that grew since they last ventured out. In these pics the white trail/area is where the Urchin ate within the last few days.

Edited by Mark Peterson - November 13 2010 at 10:11pm