Can someone tell me what this is

[asm481]

This spot appeared with another on the tail fin in the matter of two hours on my Foxface. It doesn't look like Ich to me. He is swimming just fine, hasn't sought out the cleaner shrimp I have or bumped rocks. I have been running UV on this tank for the past month in preperation for a Tang I have purchased which is QT still.
http://reefcentral.com/gallery/showgallery.php?cat=500&stype=1&thumb=1&si=PA260002part

 

[JHemdal]

asm481,

I've looked at this picture twice now, and can't quite make it out as anything truly problematical. It isn't ick (Cryptocaryon) and its placement is wrong for Lymphocystis. The quick onset is what bothers me - it looks like a typical "ding" that many fish get - but those usually grow over time as a reaction to an injury. Could the fish have gotten whacked by an anemone or a coral sweeper tentacle? If the fish's demeanor is fine (eating good and breathing less than 100 gill beats per minute) I wouldn't do anything other than keep an eye on it.

Jay Hemdal