I've seen something similar on my Xmas wrasse twice over the past 2 years. Both times, it happened overnight, suddenly and he healed up over the following few days.
I suspect it is the result of choosing a bad spot to sleep in. They sleep under the sand. Possibly they come into contact with a pocket of nastiness in the sand bed. Just my theory.
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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank: http://www.canreef.com/vbulletin/showthread.php?t=28436
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