coraline algae is pretty much there first choice but they don't seem to put much of a dent in the overall coverage. In fact I have heard that they actually help to spread coraline algae around.
The problem with urchins is that they will move everything around. I had an urchin that hitchhiked in on some rock that looked a lot like the one in the picture. It grew to be soft ball size and could move (flip over) an 8" by 8" rock no problem. It's idea of aquascaping and my ideas were at odds so it had to go
