Ok, in process of SLAMing. Balanced pH to 7.2, then Alkalinity to 110. Brought chlorine to 14 just in case it's a harder to kill algae than green. Have backwashed twice. Brushed twice. Chlorine dropped to 13 in 3 hours.
Aaaand, the red scum is now turned black. Fading, but really hard to scrub off of the waterline.
Do I treat it as black algae now that it has changed from brick red to black? Green water changing to blue.