Tang has dime sized white splotch over night?

So last night by big blue tang was fine, eating fine, swimming fine, nothing out of the ordinary.

Today I wake up and go to feed them and he is still acting perfectly find but has about a dime sized white splotch on his side? He has had HLLE for a while just on his head, I got him this way from someone for really cheap so I figured maybe I could feed him better and possibly cure it but now this just randomly happened...

I feed him flakes, mysis, garlic soaked nori, and occasionally he eats a little minced garlic.

 

Do you have any corals that could have stung him? Sounds like that to me.

 

I have two rbta's and I noticed I had about 4 hitch hiker ball tip anemones. The ball tips are very small but I have read they have a decent sting. This could be possible.

 

Where is the splotch exactly?

 

Middle of the body toward the bottom

 

Sounds like it got zapped or rubbed the spot in its sleeping area.