Having the live rock in your saltwater aquarium turn white is a common problem. Many things can contribute to this trouble, but a lack of iodine and calcium or overexposure to light are the most likely causes.
Live rock is composed of the calcium carbonate skeletons of coral organisms, with many forms of microscopic and macroscopic marine life living on it and inside of it. Live rock becomes the main biological filter of a saltwater aquarium and provides shelter to the creatures in it. You also make your aquarium more attractive by using live rock, which is why seeing it turn white is a concern.
Iodine and Calcium
Iodine is what gives corals and coralline algae their color, and calcium provides their calcareous structure. If you don't keep your aquarium provided with these two minerals, you may begin to see your live rock turning white. But simply adding them is not enough, the timing and how they are introduced are also important.
If you have a protein skimmer, it will remove any added iodine in 24 hours, and most times remove other additives as well. For this reason, it is important to use time-released iodine or turn off the skimmer for a period of time to allow the inhabitants to absorb the iodine. Many aquarists highly recommend Kent Marine Iodine products, especially if you need the time-released type.
Using Tap Water
Using tap water can contribute to developing white on your live rock. When you do water changes with tap water, you need to use a de-chlorinator.
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 adding more nitrates to your aquarium.
In a reef tank, the optimal amount of nitrate is a level that is too low to measure. In other words, zero is best. However, 0.25 ppm, but not more than 5 ppm is acceptable. Live rock, live sand, or a combination are used to control nitrates, but live rock can turn white if nitrates are too high. An under gravel nitrate control unit may help keep things in balance.
Your live rock aquarium needs more light than a tank that only has fish because some macroalgae need light in order to grow properly. Coraline algae, for example, depend on getting the right amount of light. But live rock and corals need 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 underwater and usually only get a diffused amount of light. You need to mimic these natural conditions in your tank. Don't leave the lights on 24 hours a day. Try 12 hours on followed by 12 hours off, as well as changing to 50/50 fluorescent bulbs or very efficient LED tank lights.