Hmmm...
Tough call from the pic but my first thought was Amyloodinium, or Marine Velvet.
That's tough to treat too. Would have to be treated in a quarantine tank, for sure.
I googled and found this article: http://www.peteducation.com/article.cfm?c=16+2160&aid=2428">http://www.peteducation.com/article.cfm?c=16+2160&aid=2428</a>
Not sure I'd agree that copper is the best treatment though... we've used Rally (by Ruby Reef) with some success.
IME all the fish need to be treated because some will carry it but not show symptoms. This stuff can move fast and cause major wipe-outs :(
Here's info on Ruby Reef Rally: [IMG]http://www.rubyreefinc.com/index.php?option=com_content&view=article&id=51:rally&catid=35:documentation&Itemid=56">http://www.rubyreefinc.com/index.php?option=com_content&view=article&id=51:rally&catid=35:documentation&Itemid=56</a>
Again - any treatment MUST be done in a quarantine tank, and I'd recommend treating all of the fish so you don't have recurrences.
Jenn