I'm sure there are many unknown causes for color loss, but IME when I've had this happen and the lighting hasn't been changed recently, it seems to be caused by my adding to much carbon/GFO to the system, or to little food for the corals (not feeding the fish enough/not enough fish), or maybe alkalinity/temp swings.
Elevated PO4 causes browning out, but bleaching? I don't think your problem has to do with PO4 levels, it's more likely something else.
Stay on top of regular water changes to eliminate the unknown factor, maintain your skimmer regularly, keep feeding your fish daily, add mexican turbo snails/hebivores as needed, and if you deem it necessary, maybe get a hanna checker for PO4. Also keep your alkalinity stable, and importantly don't add to much carbon/gfo to quickly, that alone can cause coral bleaching and tissue recession.
This was one thing it took me a while to figure out, sps need food in the form of fish waste. I was always scared of getting algae blooms, but IME corals need nutrients to maintain color. it's a balancing act isn't it?
