pH and Alk are affected by each other but are not the same thing. Alk measures the capacity of your SW to buffer against pH swings. I don't even pretend to know chemistry but I know keeping your alk where it's supposed to be is very important.
AS for the coraline alagae, there are many different species and the one that is now covering your rock may not be finding the conditions in your tank to have what it needs to continue to grow. Usually, as one species is dying back, one or more other species will be able to find a niche and start growing in your tank. You'll see different colors of coraline, maybe even some bright green or orange. Your original coraline may even stage a comeback after an adjustment period. It's a surprise
It may take a while though, so be patient.
I wouldn't worry about adding calcium, either, until you have corals and other animals in the tank that are more calcium dependant.
Alice
Instead of just building a reef in my home...I so wish I could afford to build my home in the reef! 
"A BRW Original"
