Picked up a tuxedo urchin from Ocean City today. I wasn't 100% confident that they would go after my specific kind of algae. It's been in there for about 4.5 hours now and here's what it's done:
It's carved a clean white path in the rock that it's passed over. I have to say I'm very pleased. It's too early to tell if this is the answer to my problem, and I'm sure I'm going to have to learn to live with it re-arranging my frags (it's already tried to use a hermit crab as camouflage), but I'll keep updating as I go.