So I guess as long as the temp is right and stays right, it might be stress. I had the same problem with my first set of chicks, and they came out of it all right. I actually didn't clean it off at all (wasn't sure if I should), I would just check and make sure the vent was not glued shut. Just...
http://home.centurytel.net/thecitychicken/broodchicks.html
http://www.strombergschickens.com/stock/chick_care.php
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http://extension.missouri.edu/explore/qa/poultry0003.htm
"Pasting-up" of the rear of baby chicks (feces sticking and drying below and then sometimes accumulating...