Excellent tank if it was mine I'd simply lower light intensity over the uncalcified areas, purple ones don't take on as much algae they compete against it, use spot treating to clean off each frag outside the tank and put back clean, drain the reef for a 60% water change and I'd hand scrape and remove all algae off the back wall until fully restored. I'd spray that back wall area with peroxide to burn off leftovers, wipe it all out really good with paper towels and fill back up.
Algae that grows on the glass indicates a water transition phase in its cycle
For sure if you had giant oversized uv, you would be working against grow back. Not required, but a handy cheat for transiting algae
The sandbed looks clean and not typically pocketed w waste, not a huge fish bioload. Lucky it's mostly on the wall that's really nice reef rock. IMO the sandbed contributes only minor to the issue it seems to be a balanced scape to me. Target control needed, and big exports are a breath of fresh air the corals extend next day better than ever.