A Live Rock turning white is a common problem that could be contributed by many things. LOack of iodine, calcium, and overexposure to light are the most likely causes.
Iodine & Calcium
The iodine is what gives corals and coralline algae their color, and the calcium their calcareous structure. If you have a protein skimmer, it will remove any added iodine in 24 hours, and most times other additives as well.
For this reason, it is important to use a time released iodine, or turn off the skimmer for a period of time to allow the inhabitants to absorb it. Many aquarists highly recommend Kent Marine Iodine products, especially if you need the time released type. To learn more about this topic, refer to our Aquarium Care Resources Index.
Using Tap Water
When you do water changes with tap water, do you use a dechlorinator? If you haven't, start. The chlorine in tap water is toxic to all of your aquarium inhabitants. Another reason why you should not use tap water is that it can be a source of actually adding more nitrates to your aquarium.
In a reef tank, the optimal amount you want is an immeasurable one! In other words, zero is best. However, 0.25 ppm, but not more than 5 ppm is acceptable. An under gravel Nitrate Control.
Live rock and corals needs to sleep too. You have to remember that even in the ocean these inhabitants only get about 12 hours of light in a day.
They are also under water and usually only get a diffused amount of light. You need to mimic these natural conditions in you tank. Don't leave the lights on 24 hours a day! Try 12 hours on/12 hours off, as well as changing to 50/50 fluorescent bulbs. For more on this topic, refer to our Lighting Resources.