Okay I have a ton of this stuff. Its a long story so I will keep it short. Before I got my "free" tank it was over run with this stuff, I mean like golf ball sized BA. I have learned a lot about it. One you cant just pull out the rock and scrub it. You will need to take some of the substrate with the BA when you remove it or it WILL grow back. I have tried to pick it off and it just grows back, you NEED to remove all of the foot or hold fast. I have been very frustrated with this stuff to the point of wanting to start over with a new tank and rock! SO after awhile I decided that it didn't bother me and when it got bad I just ripped as much off as I could in the tank then sucked out the left over bits. I thought this might cause a even more major outbreak, it didn't. The BA did not spread, I have some rocks that have none on it and others are covered with it. It is now so bad that some of the older pieces of BA are covered with coralline. This seems to kill it after 3 or 4 months and it does not grow back. It also does not like to grow on coralline, but seems to thrive on bare rock.
So if you have only a few spots of it try to remove the rock and chip off part of the rock with the BA. Emerald crabs do work in my case I would need 2 or 3 hundred of them. Really! I currently have 5. If you don't mind crabs I would put in at least 5 to control any further out breaks, they seem to like the smaller pieces of BA. When they get to dime size they wont eat them.