Nitrogen Deficiency or Light Burn.

Doesn't look like nitro-tox. Curling down is a sign of too many nutes, though. I wouldn't worry about the single-bladed leaf, but the one on top shouldn't do that, yet, even at a younger age when things like this can just happen.... it could just be a bit sensitive to the nutes and grow out of it without changing a thing. Nitro tox will have dark leaes that start at bottom and darken upward over time. Several things will cause dark leaves. Too much light will cause nodes to stack on top of each other. The leaves will look oddly close together, etc... Top leaves would begin to fade to white over time. Simply raise 2-4" and see what happens. If it is light-cause, it's not severe and a few inches will easily fix it. At that size you want the lights pointed at the few leaves you have from the top at the distance they likely recommend. as it gets bushier go ahead and see and side ligthing helps -- if that's a normal configuration in the picture. Light doesn't do much if not direct at tops of leaves, and if only a few, go from the top only. If it's pointing at bottom of leaves, it could be increasingn evaporation, which could cause some leaf cupping, maybe? Potentially some nute burn in tips of leaves. I'm just not certain with that sort of light. 12-18" on lights? umol/s can help figure it out, if it is accurately rated. That's a 50-true watt light? it's likely good for 1 sq. ft of space and may not be enough for blooming a plant properly. Watts are a horrible measure to use, but probablymore correlative than their par or umol/s spec. Par is a per m^2 measurement based on total umol/s. So if oyu take their umol/s rating and use some algebra to translate 1m^2 to your area, you can determine your relative PAR or ppfd, whatever the correct vocab is. ~100umol/s in 1sq ft is a fairly high par rating, for example. Small space, lots of photons. This plant is also a bit young. They get more robust as they grow a bit more, if it is a senstivity to nutrients. It's possible you can do nothing and it will be perfectly healthy with zero effort soon. "organic" grow also means you have no idea what's going on in the soil chemistry, so it's best to wait until you are absolutely certain, although moving the lights a few inches is simple and not a huge impact or stress. You can easily re-adjust the moment you notice too much stretching. If nodes are spaced well, it's not the light.