[ChuckG]

Without researching (which I don't have time for) I remember something to the effect of a high calcium level contributing to the effect of turning an aragonite substrate into a solid block of concrete. This happened to me once and I "think" I was told that it was a high calcium level that fused my substrate together.

You will see both opinions concerning coralline and you are correct that nuisance algae will have a harder time attaching to coralline than to just rock. I believe MDP even says that in the long run it is more of a nuisance algae than a benefit.

If live rock is your filter and water needs to flow in and around this rock to be effective then what happens when the rock's pores get completely clogged up with coralline growth?

Will your aptasia come back if you stop dosing purple up?
Who knows - only one way to find out.