If you took that rock out of the water for more than about 30 to 60 seconds then you killed the coraline but it will grow back. That is the most likely explanation for the bleaching coralline.
Also, you learned a valuable lesson. The fish store is there to make money. So if your fish dies during the cycle, then they will be there to sell you more. Sometimes the best thing for the fish isnt what they tell you.
I agree, that it is just coralline that has died off from being exposed to the air. This is nothing to worry about, and nothing you can fix -- it will come back with time.
You will need to upgrade lights if you plan on keeping corals. The lights you have are fine for keeping fish only, but not strong enough for photosynthetic animals.