That is good but try not to let the FC go below 15ppm.

Shock levels will give falsely high pH readings - at 7.9, I wouldn't try to move it until the shock is over and FC returns to normal. I also wouldn't move CYA until the shock is over. Some people in warm, sunny areas like to run the CYA up to 90 to reduce total FC consumption and the frequency of adding chlorine - just remember to raise the minimum FC to correspond.

The black stuff may be easier to brush away after it's dead, just try to brush it enough to keep it exposed to the shock level water.