I agree it sounds like possible skin that hasn't shed yet. When in the water the water would cause it to 'float' so it's visible, but while out of the water it would drain and not be seen. If so can be helped off by gently pulling on it.

Hope that's it, it would be an easy fix.
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**Kenn**
The Uros at home:
http://www.arabianwildlife.com/current/dubh.html
http://www.tortoisetrust.org/articles/uromastyx.html
http://www.fortunecity.com/greenfield/runningbrook/729/id19.htm