Sounds like columnaris but, can't say for sure without pics. Follow the recommendations already offered. If it turns out to be a columnaris infestation, here's a regimen that works to clear it up:
Columnaris Treatment
Copper sulfate has always been the common treatment. But, copper can be just as damaging to fish as the disease. I've found Terramycin and Trypaflavin can do an effective job on columnaris.
1. Pull carbon.
2. Gravel wash and water change to reduce pathogen population.
3. Terramycin bath. *(see below)
4. Add salt to the water to aid gill functions.
5. Treat food with Terramycin to fight internal infection. **(see below)
6. Treat tank with chloramphenicol + acriflavin (trypaflavin) per dosage on container.
7. Gravel washes/water changes every other day. Replace salt and meds.
8. Treat for an additional 10 days after symptoms are no longer present.
This has worked for me on fresh and brackish fish. I haven't had the opportunity to try it on marine species (knock on wood). But, I've read of others using this treatment on tangs.
When fish are infected with this pathogen, the following signs can be anticipated:
Skin
There will be necrotic lesions on the skin, which often are white/gray coloured with an edging of red. These will quickly in one to two days, transform into ulcers with have an orange/yellow colour, caused by the bacteria decaying the underlying tissue.
Gills
Similar effects very typically occur on the gills, but may for the average hobbyist be somewhat harder to observe at least in the early stages. The progression of these ulcers, causes the fish to have great trouble with its respiration, and thus can quickly lead to fatalities. If the gills are examined, excessive amounts of mucous, are to be expected.
Behaviour
The fish will become very listless and lethargic, often will hang at the surface, trying to breath there, although on occasion, the fish will rest on the bottom of the tank. Reluctance to feed is very typical and the fish will become anorexic. Respiration is often rapid, as the fish fights to overcome the damage done by the infection to its gills.
Body
In some cases, the lips of the fish, will become swollen and macerated, and a milky slime like film can be observed with the naked eye on parts of the body.
Fins
Large milky patches can be seen quite easily on the fins of the fish, and this is usually an indication that the disease has progressed to a degree that cure will become much more difficult. One typical sign is the appearance of a "saddle" shaped lesion usually around the area of the dorsal fin, and this occurs so often, that the name "saddle back disease" is often used in aquaculture to describe this infection.
*A "bath" is an immersion treatment with concentrated medications. Usually a small vessel (like a bucket) is filled with tank/fresh water and dosed with medication (following bottle instructions). Once the meds are diluted evenly within the bucket, the affected fish are netted out and placed in the bath for several minutes (again...according to the med instructions).
**Foods (dry/frozen/fresh) can be treated with medications to target internal infections in a fish. The meds (usually liquid but, can be powder) are added to regular foods (per med instructions). For a disease, such as columnaris, internal medication is important since the disease can cause gill damage/necrosis.
top pic: fishjunkies.com
btm pic: Herman Prosje