it eats the other remaining food left on the sand bed.. in other words its also helping u to clean up ur sand bed.. take note of the middle top part... u will see it clearing some stuff out and thats his **poo**
u may keep it.. its harmless.. i'm also having 1 in my tank..
however Quotes by Bro jerv remove it when u sense something is wrong with the sea urchin.. and 1 more point nv get poke by it.. will cause serious pain on the wound