I had a much smaller issue with hair algae and I tried three different urchins over the years and all they wanted to eat was my coraline algae. I then got a sea hare, and it wiped the algae out in a few weeks. It would probably take a bit longer for you, but something I'd consider over a pickie urchin.
Some of our friends may know of a reason that wouldn't be a good choice for your situation though.
Some of our friends may know of a reason that wouldn't be a good choice for your situation though.