Hi Glenn,
I recall you asked about this earlier.  I have never heard of such a thing.
Do you know if the shell in question is supposed to be a bivalve or a
gastropod (snail type) shell?  I am wondering if it might be the mussel
Ryenella impacta (Hermann) (sometimes called Modioloarca impacta).  It's a
common, shallow water New Zealand species.  Beach specimens often have a
pretty green color, but I don't know if it has anything to do with exposure to
sunlight.
Paul Monfils
Providence, Rhode Island, U.S.A.