I has it!! Lowered pH to 7.2, raised FC to 20 ppm, brushed the offending sites. When generic SLAM is done, I'll raise FC to 29.4 per the FC/CYA chart by ChemGeek and hope it's GONE!!
I had to resort to the mustard slam recently as a SLAM a while back didn’t hold even though I passed the tests. Never saw any algae though. Just chlorine consumption and a hint of cloudy water.