Less light - it can't process light as well with fewer zooxanthellae in it (it expels them when it bleaches). Feed it meaty foods 2 or 3 times per week, if it will accept them that often (if it's not hungry, it won't eat). As long as its tentacles are still "sticky", and it can grab the food, feed it up and eventually the zooxanthellae will multiply within its tissues and it will colour up again.
Pink is usually a sign that a red/rose anemone has also bleached (=expelled some of its zooxanthellae).
I've nursed a few back to good colour from bleachy white.
Jenn