This is most likely what I've described as a bacterial infection. Once the first or second layer sheds, the turtle is fine. Here is a more recent topic:
viewtopic.php?f=5&t=34190&p=324901&hilit=bacterial#p324901I would do very close and regular examinations of the shell. Firm, hard, no odors, no sensitive spots, no discoloration, no pits, no deep shedding, etc.
Has there been any changes to the tank prior to this? Decorations, additives, plants, lighting?