Large Spot on Yellow Tang- not ich

[Stoph70]

I had a large discoloration spot (half of one side of its body) develop on a yellow tang overnight- what is it!?

 

[blazend]

Not sure exactly what it is but my foxface came down with a white spot (just 1 large one) and i had to remove the carbon from my filter and within 3 days it cleared up.

 

[scooter31707]

If possible, a picture would be helpful.