It looks like it was Urenema marinum. Happens a lot with Chromis, possibly because of the stress of shipping, and shipping method. Often when these fish come from overseas, they are packed with many in the same bag, and the water fouls during transit.
http://www.advancedaquarist.com/2007/10/aafeature1">http://www.advancedaquarist.com/2007/10/aafeature1</a>
Scroll most of the way down the page to find the write-up on it.
The best way for the hobbyist to avoid this, is to only buy fish that have been landed for a while (ie - been in the store for several weeks). Usually that ailment becomes apparent within a few days of arrival. We never had a lot of success treating it - once you can see it like that on the fish, it's usually too late.
Jenn
We used to call it "herpes".