Why not get a few more urchins to eat/pull the algae off the rock? If you have one that is doing it, but just not keeping up, give a couple more a try. It will save you the grief of doing it all yourself, and personally, I love the look of urchins. Such a cool animal. I know it is doing things backwards, not finding the source of the problem, but it can be satisfying seeing them tear that algae to shreds.
I know it was for me.
