What soil is that, and what have you been giving them? The yellowish bottom leaf I would guess is from touching the soil, so I'd just pluck it off. But it almost looks like bugs and maybe not enough aeration in the soil. More info will help though.
. Didn't even realize I've already posted here, and you already said FFHF, 
14 days from sprout. In 4 inch pots with happy frog potting soil Under 600 watt MH. Have only given them water. Temps and rh under control and in range.
Here’s a bunch. The last one is to show others in same condition that look fine
Ya, I was one of them, but I'm a loadie,
. The only yellow is the bottom leaf right? Could it have been touching the soil? I'm not a big fan of neem oil, but if you're doing the organic thing neem meal helps with some bugs. There's also predators.
You going organic, or bottled nutes?
I'm actually gonna test the difference between the Millard and the Ball stuff. Just haven't done it yet.
Yea dry amendments, but I haven’t given these plants anything yet . Just sprouted in happy frog potting soil 2 weeks ago
Mulch it. It'll help keep your soil moist, and more microbe friendly. Maybe give it a compost tea as well, or use some recharge. Get some fungal activity going as well.
Barley or Bokashi is good for fungi too.
I recently heard mustard seed is really good for fungi also.
Wow. That's a new one to me.
Wait, I could be wrong on that. Let me watch that again.
Xtsho provides this 3 teaspoon per Us gal.food grade