live rock is not really alive. it's the bacteria that resides on it that makes it "alive". turning white could be a few things:
some times folks upgrade their lighting which kills all the coralline, and the rock turns white.
sometimes they have an urchin in the tank that eats everything on the rock rendering it white.
sump rock that doesn't see any light is usually white.
some times it's just going through a stage. for instance, you buy live rock with all sorts of stuff growing on it from the lfs
. put it in your tank and between the transport out of water or wrapped in damp newspaper, and the cycle, all that colorful stuff dies off and white rock is left. this is usually the case for new tanks.
that rock you get from the lfs
sits out of water for days before they get it. damp from a sparse coating of wet newspaper- hardly enough to sustain some of the life on it. then you put it in the tank. all the dead stuff causes a cycle as it decomposes. ammonia kills more stuff....white rock is what remains.