Question about my Hippo Tang...

[George]

I think it's ich. Ich (or the symptoms of it) can come and go as you described if you don't treat for it.
Your other fish don't have the symptoms most likely the fish have immunity to it already or sometime fish have ich but they don't show the symptoms (white dots).
Of course the best option is to treat all fish. I know this is not something for everyone so leave them there for now is another option. Just feed them quality food and feed them well and they will be fine.
Coral burns look very different from ich.