Rock anemone is quite a general term and does refer to a few different species. Do you know the proper name or possibley post a pic? White can sometimes mean it's "inerds".
This kind of behavour is usually from some sort of environmental stress or possible from nutritional problems. Could you provide some additional info about the tank water specs, it's set up and other fish/inverts in the tank as well as how you care for the anemone?
Cheers
Steve