A skunk cleaner will definately help and/or cleaner wrasse. There is also the dreaded freshwater dip, but that can be quite nerve-wracking, on you as well as the fish. But it does work, tangs are a bit more touchy to this procedure due to types of bacteria that live in their stomachs for digestion purposes, so use caution and research before hand. I would first check your water quality and temperature. Stress is the number one cause of ick. Generally it comes from temperature flucuations, but could be a wide variety of variables at this point. Did you have someone topping off your water while you were gone? Changes in specific gravity will also bring ick on. Have you introduced anything new the aquarium? I guess pretty much what I am asking is...what has changed?
I would recommend a quarantine system, as jd suggested, but would also recommend not using copper based or formalin based medications on tangs as this is also hard on them and will most likely kill them. I like to use Marine Melafix or Kent's RXP treatment, as these are derived from natural ingredients as opposed to poison.
If you go the quarantine route, get a small tank, a heater, 3/4 fresh clean saltwater 1/4 water from tank, a power filter with no carbon insert, and an airstone and pump for oxygenation. No gravel, decor or light. All of these items are very important, but raising the temperature is key. It speeds up the life cycle of the parasite. Follow directions on all medications, very important, if the ick goes away in a few days and the meds call for a seven day treatment do not remove the fish from the treatment. While in the quarantine, you can run the specific gravity at 1.017 this will also keep the ick from being able to reproduce. If you want we have several used tanks and meds, probably have some used heaters and filters if this is the route you decide to go. We are located in Troy if you want to stop in.
Saltwater ick is a very difficult parasite to treat.