What is the location of the BA? Are you able to remove the rock it is on and manually remove the BA? I had gotten a frag that developed some BA and it eventually spread to an adjacent rock. In the end, I found that manual removal worked the best. When going with the manual removal method, be careful not to rupture the balls as it is my understanding that it can release spores that can result in the spreading of the BA. But if you do, my recommendation would be to rinse the rock in SW before returning the rock to the DT.
As for emerald crabs, in my experiences with emerald crabs they are hit or miss whether they will actually eat the BA and with the ones I had, they were all misses.
I hope that helps,
Tom