SPS help

[Gordoe]

Looking for some suggestions to help my corals.
My SPS corals are dieing off slowly, one by one. Parameters are all ok. Fish inverts and softies all growing well..

The corals start to go white in one little section and eventually spread over whole coral. Is this a critter in my tank, or bacterial infection?

I'm ready to pull them and put them into a new tank, but I don't want to infect??

Calcium is hovering at 300-400ppm. If this is possible the reason, how quickly can I bring it up to 450ppm?

TIA