Can you wipe it off with a Q-tip and alcohol then increase the air circulation around the plants? I've had something similar on seedlings I've kept the humidity up on. After the increased air circulation, the plants were fine.
Joann
I kind of assumed it was a type of fungus, but I didn't repot the seedlings so I also assumed there were no spores in the potting media. So....nope, I don't know what it is.
If it comes back, I would probably use a Q-tip with brown listerine to clean the plant and repot it in fresh media though.
Joann