Agree with ccccCccapt... your calcium/alkalinity are fine, but those phosphates could be an issue.
As you're noticing, it doesn't like a lot of light. It won't grow in areas that have no light, but it will do best in shaded areas out of the direct light. My tank tends to have less coralline the higher in the tank you go... and I have power compacts for lighting - not metal halide. Again, as ccCapt mentioned, I think the high output of your lighting is doing in your coralline.
Not sure about the blenny eating coralline... maybe it was picking at algae that was growing on it, or some pods
that were hiding on it?