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Could be an encrusting algae that is dying off...I'll assume your tank is cycling and that would easily cause such to occur. As rock matures through the life of a tank, it takes on all kinds of colors, but only algae that is why is dead/dying. It isn't really that big of a deal.
If it was actually live rock and kept wet it could be ok, though I always want to add an ammonia source to make sure that there is enough beneficial bacteria to support livestock.
I've just added 2 clownfish today. All the tests were fine and I took it down a sample to the fish store and they confirmed it was cycled. The live rock has a lot of algae growing on it. Just turning white a bit.
It may have been a mistake to add rock 2 weeks into the cycle, and live sand doesn't have enough die off on it to cause a cycle at all. Poor advice I'm afraid.
You may want to dump in some of that bottled bacteria.