Birdsnest turning white

It's dying. But by now, you already have that figured out. In the future when you see that happening, you can frag off the tips or parts that are not affected. The necrosis spreads under the tissue and can wipe out an entire colony. I have saved several colonies by cutting off the affected parts. In a situation like yours where the base is infected, you will need to cut off the tips/branches to save them.

Check your parameters such as KH, SG, Temp and make sure it stays steady within an acceptable range.