Ok I am not an expert...but to my knowledge Ich can be brought out(for lack of a better term) is fish, by stress, poor water quality. I was told it is something that is always present in their systems, but just comes out when they get stressed. Kinda like the cold in our bodies.
Cleaner Shrimp will help take care of it natually as will garlic oil.
But it can kill fish if left untreated...Make sure if you get ich meds from LFS that they are saltwater safe and ( if you have) invert safe. Many meds will kill all inverts.
Hope this helped!