it's frag surgery time. The water isn't the cause, the exposed coral skeleton is home to anything that colonizes and algae landed first.
I just prepared this lps in the same way:
Exposed septa is attachment point
Lift out coral work outside tank, water wasn't a problem, opportunity was the cause
Use tweezers delicately pull off algae, use q tip dipped in peroxide wet, dot the cleaned skeletal area.
Put back in tank clean. Polyp inflates over cleaned within two days and if it grows back, repeat. That big old coral may take time and feeding to add back mass. It's wound care, algae was the infection
Debride it
Smaller invasion below but still a tuft of algae on the exposed septa.