Ah, thank you both. A little googling shows the soil mite seems the most likely to what I'm seeing. They're very mobile little things, hopefully I can get rid of them but at least they're not killing the plants in the meantime. This is a little odd, mind you, since I did think they came from a new ...

Indoor lights will risk etiolation if the plants are well established already, unless you have some really beefy LEDs to throw at them, it's not really worth it imo.

Dark and cool is good to go.