I have a tuxedo urchin in one tank and a long spine in another tank. I got both of them as freebies when I was buying other stuff from people getting out of the hobby.
They are really neat creatures and very interesting to watch. They eat nuisance algae better than any snail.
If you don't mind them eating coralline algae (100% solid purple rocks are not natural anyways), or knocking things over that are not solidly attached, then they are fine.
Since yours is in the sump, it won't hurt anything. You can transfer rocks with algae down to the sump for it to feed on.
Best of luck,
Roy