http://www.ecosystemaquarium.com/products/marine/miraclemud/miracle-mud/
The owner SY Leng have been runing different configurations of tanks for years and months, he always updated on his website.
Best result is using both mud and skimmer according to the test if I remindered correctly.
It is just a minor external bacterial infection.
Feed the fishes with garlic and vitamins. If its condition does not improve, just do a 20% water change.
If become ulcer ... do some drips with yellow powder.
Just add a larger emperor than existing blue face. Normally, the emperor and blue face are shy and seldom fight with different kinds as compared with koran, six bar and blue ring. Will tender to fight their own kind.