dead coraline

[Snappy]

I tested my parameters today and found my alk a little low (7dkh) for what my overdose in calcium was (ca about 530) so I did a water change and a few adjustments so things should sort themselves out in a day or two. I noticed when I got back from holidays earlier this week my coraline algae was mostly all white. The corals all look fine but most of the coraline is dead. Could that be from the kid taking care of my tank while I was away over dosing? Any other ideas as to what would cause this?