Your description is very good. I'm leaning towards infection, and maybe you want to treat with an antibiotic. Certain fungal infections on fish tend to me more raised and fluffy in appearance which isn't what you are describing in the least.
Do you know what the previous owner fed it? If I were you I'd add in that extra airstone, and probably start treatment with an antibiotic. In the mean time I'd maybe try clams on the half shell, nori on stick (my triggers in the past loved this), maybe live ghost shrimp. Its also possible that the fish just isn't hungry because of the infection.
Your doing all the right things, and I really hope this pulls through for you.
This is from liveaquaria about fungal infections in marine fish; "Most fungal infections have a characteristic white fluffy appearance and are commonly known as 'cotton wool disease.' As the fungal infections worsen, they may take on a gray or even red appearance." Since this only happened in about a week I'm doubting its fungal. (
link)