Ok, from my experience there is no such thing as a healthy white anemone, least not in the ones we commonly deal with. Part of the thing about sebae anemones is that when they are collected they very often, and easily loose there zooxanthellae. This is cause for some concern, doesn't have to mean that it is doomed but at this point we really need to give it the best enviorment possible to bring it back to proper look and health.
Anemone Bleaching
What is anemone ‘bleaching’?
All healthy anemones have a population of zooxanthellae in their body tissues that helps provide nutrients to the host anemone. This algae population is brown in color, and all healthy anemones will have an underlying color tone of light to dark brown depending upon the intensity of lighting under which they are kept, and the size of their zooxanthellae population. In cases of stress or poor water conditions, the zooxanthellae population in an anemone can die or be expelled. This condition is called “bleaching” since a healthy, dark colored anemone can become light and pale overnight. Bleached anemones are white or translucent in color, and lack the noticeable brown undercoloring of healthy anemones. The lack of zooxanthellae puts an anemone at significant health risk, since it no longer has a substantial source of nutrients. However, even badly stressed and bleached anemones can recover if placed in a stable, healthy environment. Daily feedings and appropriate light conditions will help the animal to rebuild its zooxanthellae population and reacquire its normal tan/brownish color over time. This process may take a while – three months is not unusual.