Tangsfornuttin,
You can stress out clams switching from different lighting schemes, ie VHO to MH. A lot of the vibrant colors in clams like maximas and croceas seem to help protect them from UV rays. If a clam has been under low lighting it will increase its zooanthalle (sp?). This makes the color fade and the clam will turn brown. This lets them absorb more light, but at the sacrifice of UV protection. If they then get put then under intense lighting, they may start to bleach. If you don't know what lighting the clam has been under, or you are making a big lighting change, it's not a bad idea to adapt them to your lighting slowly.