I dont think you should be worried about it all. That looks like the cotyledon (seed) leaf. The plant looks really healthy and responded well to topping. Could be several things. just from being close to the soil. being the seed leaf like I mentioned, they tend to die off naturally. Although in healthy plants not uncommon to see them well into veg. looks kind of like wind burn too, but the other leafs are okay so probably isn't it.