Leptastrea coral turned white

[Oggey]

One of my Leptastrea coral has turned white all down one side. Other Lep and corals all open and look good.
Checked parameters all good except Alk down to 5.4 but see that a lot and dose weekly to raise.
Day after whitening APEX alarmed with PH LOW below 7.8, dropped to 7.4
No light changes made and verified still have settings.

 

[Fish Think Pink]

I try to maintain pH between 8-8.4 ... your settings for pH seem very low to me