It's white because it's sick. More specifically it's bleached, meaning most if not all of its zooxanthellae have died and been expelled. Zoox is required for the anemone to survive as they have a symbiotic relationship:
"Sea anemones position themselves in a way as to increase light exposure to their symbionts. When their tentacles are relaxed, the algae lie in a single layer. When the anemone contracts its tentacles, the gastrodermal cells shrink and the algae lie on top of each other. The zooxanthellae benefit by receiving carbon dioxide from the respiration of the host, nutrients such as nitrogen and phosphorus from the hosts metabolism which are then recycled back and forth between the host and the symbiont, and a shelter in which to live. The sea anemone benefits by receiving oxygen and food in the form of glycerol, glucose and alanine from photosynthesis."
A healthy gigantea typically has a brownish green tinge, with another dominant color -- it could be purple, green, blue, tan, or multi-color. A typical purple gigantea looks like the one in my avatar.