Is there anything to feed them to keep them away from the coralline?
Try feeding then little nori strips. I have 4 that have been in my system for well over a year with no issues.
If they do munch a bid of coraline, don't sweat it much. They will poop it out and it will spread even more.
urchins have had bum rap as far as depleting coraline, I think it is much overblown..
Very entertaining animals IMO
If you are concerned about the number of them in your system, take all but a couple to your LFS
, I'm sure they could find homes for them.