James,

Sounds like you left it in the solution too long and the acids from the
decaying animal clouded your shell. As far as I know it is permanent. NEVER
ever leave a Cypraea in any solution for a period of time unless you know all
the animal is out. Often you will see dull spots on some shells that would
otherwise be gem. It is almost always due to poor cleaning and the decaying
animal splashing onto the shell. That is why you often see expensive Cypraea
with cotton or toilet paper in the aperture to prevent that from happening.
Sorry, it's happened to most of us. We learn the hard way.

Frank