bleaching coraline is pretty common in new tanks. it could be because of the light, the ammonia spikes, calcium/alk
not being balanced etc... not all of it will die off though and once the tank is more established, it will come back.
make sure you test for calcium and alk
before adding anything. usually water changes will take care of it and keep the levels pretty stable. if you are going to add anything, i would recommend getting a 2 part additive. its just too hard trying to balance the 2 on your own. HTH