Spot on PB Tang

[mudbug]

Hey all. I have a spot on my powder blue tang. I guess its about 3/4 the size of a dime. I've had the tang in the tank about week. At the same time I also added a Majestic angel fish.
He appears to be healthy otherwise. I have noticed him rubbing the spot on coral every once in a while but he swims around and eats voraciously. I feed the fish nori and one cube of frozen brine a day. All of which they eat very quickly.
I'm FO and I'm skimming, running UV, and have a titanium ground probe in the sump. Ammonia=0, Nitrite=0, Nitrate=25, pH=8.2, temp=78, sg=1.023.
If anyone has any ideas, I'd appreciate it. Thanks.
110g FO
PB Tang
Majestic Angel
Three-stripe Damsel
Scooter Blenny

 

[mudbug]

Doh! I forgot to upload the pic. Here it is...
<a href="http://www.f150online.com/galleries/pictureview.cfm?pnum=14520&anum=1422" target="_blank">Tang Pic</a>

 

[mudbug]

After doing a search on HLLE, I would venture a guess that this is what the PB has. Its not near the head which through me at first.
I add a few drops of Vita-chem to the frozen brine before I place it in the tank. Should I put the drops on and then let it sit for a couple of hours rather than just a couple of minutes? Also, should I soak the Seaweed Selects in it as well?

 

[mudbug]

Anyone?