calcium is 20 points low---significant. Ph is fairly irrelevant. Your magnesium is possibly low, letting calcium and alk fall. It's fairly critical to get this up within the next couple of days. If you are having to raise magnesium, raise it first, then raise alkalinity to 8.3 if lower than that, and lastly raise calcium to 420 or a bit better. You need the corresponding supplements, dose, wait 8 hours and test, and do not proceed to the next element until the first element is up to snuff. It's an interrelated set of elements and if mg isn't up where it belongs you can add the others for days on end and get nowhere good. first the mg, then the buffer, then the calcium.
What I use: Salifert tests for magnesium, dkh alkalinity, calcium; and Kent supplements for doses---Tech-M for magnesium (mg is proper abbreviation); DKH Buffer for alkalinity; and Turbo Calcium for calcium. Once the corals really start eating, there are ways to automate this, but it would be overkill with just one coral.
Good luck!