Just did some additional checking with a bright light and magnifying lens. Not one sign of mites. Also, once again I tapped each leaf with paper underneath, and not one speck landed on the paper - so I'm actually thinking in my case it's not mites.
So - I'm leaning towards leaf burn (which has me stumped - they've been under those lights for 6+ months, not sure why it would show up now). Are there fungus afflictions which look like this? Could it be heat related? It get to about 78F at the plants height, which seems fine to me. Those of you who have seen this on your plants - did they recover?
PS - if you look in the background on the first pic, you'll see algae in the pot. I do fertilize twice a month at 1/4 strength and flush between. So, is algae detrimental enough for me to try to eliminate?