Generally reef-safe, coralline notwithstanding. A 24g tank may not provide enough food for the urchin, especially if you have other algae grazers in the tank.
I would have to say not in a 24gal. Algae grazers like urchins need a good size tank to sustain them not to mention the fact that their spines in a small tank like that will wreak havok
thanks for the feedback would a very small pencil urchin be ok it will be the only grazer in the tank the tank up for over a year and has lots of life in it...