I think you may be trying to too much too quickly. Whats happening is that the low Ph and Alk probably cause coral stress and die off releasing nutrient into the water. These nutrients are then absorbed by algae, causing the algae bloom, which further absorbed the nutrients and sending the tank to a downward spiral. Its a competition issue now instead of a nutrient issue. I would just focus on getting the alk and PH up to 9dkh and ph above 8 to stabilize the tank. Remove the algae by manual removal instead if any commercial products which could further upset the already sensitive balance in your tank. Ur tank has low nutrient bc the algae absorbs it too quickly. Don't try to chase the no3/po4 numbers bc it will only make it worse and feed the algae.
Corals need no3/po4 to grow and encrust, but it only needs light (at least in the short term) to subsist. So, I would only focus for now on getting your ph and alkalinity back in line will stop the deterioration bc at low ph the water literally pulls calcium bicarbonate out of the coral tissue.
whats ur mag lvl? Didnt see it but if it's low that could be why ur alk was so low (due to precipitation). It will take a few months to correct since it took a few months to get to this point. Just be patient and have faith that it will as long as u keep water as stable around 8.2ph, 9dkh. 450ppm calc, 1300ppm mag. Good luck!