Tangs are nicknamed "ich magnets" and with good reason. They seem prone to ich more than any other fish we typically keep in home aquaria. IMO it's due more to the inadequate conditions that these fish are often kept in, rather than a natural inclination to be more susceptible to ich.
Tangs are often kept in aquariums that are too small with not enough free swimming space, thrown together with tankmates that cause them stress, aren't provided with an adequate diet heavy in greens and readily available for grazing and placed in immature systems that suffer in water quality from too many fish too soon.
If, however, a quality environment and diet can be provided, tangs don't seem to be affected much more by ich than any other fish. QT'ing all new arrivals, fish and invertebrates, will go a long ways to keep ich from entering your system.
As far as munching on things, tangs don't get much of a bad rap, it will probably depend on the individual appetite of the fish.
IME, yellow tangs tend to be more skittish; purple tangs more aggressive but all tangs can be belligerent and territorial.
I hope this helps
Alice