Rev- I would isolate the tang as the main culprit. However, the stress the fish has caused to the coral leads to bacterial infections that you can not see.
If anyone has an LPS coral that is receding and showing skeleton and a fish is not the cause, then assume it is bacterial and follow these steps.(no fragging required)
1. Set up a container of SW large enough to dip the coral in for at least 15min to 1/2 hour.
2. Add to this container of SW hydrogen peroxide(store bought). Add 1 capful at a time until the sides of the container begin to form bubbles. Wait about 30 seconds before adding the next capful until you see the bubbles.
3. Once the bubbles are visible on the sides of the container, add the infected coral. Dip it for 15 min watching for extreme stress. If it closes but isn't sloughing keep it in there. If the coral looks like the dip is not affecting it add one more capful and leave it for the 30 min period.
4. Rinse the coral in clean SW.
5. Return the coral to the main tank or a hospital tank if you have one.
6. Repeat this procedure each night until the coral begins to recover and inflate properly.
This method also reverses the affects of a brown jelly infection. When treating for brown jelly, use a soft toothbrush to brush away infected tissue. Then do the above dip and make sure it is rinsed well. Repeat daily until signs of improvement occur.
The above treatment usually takes up to 5 days.
Water changes are always recommended as they always help. :)