It's been said in a couple other threads in the reef forum that, Coraline turns white when it is dead/dieing. This can happen when your magnisium is low . If you are using Instant Ocean salt mix, they took the magnesium out of the mix, thus you wouldn't have the magnisium you would if you used a different salt mix. Might want to add a Trace Elements Additive.
As far as the diatoms, make sure you use Distilled or RO water. If your tank is fairly new it 's normal to have them and will eventually go away on thier own.