What could this be?

@Med_in_Tropic You may be right about the excess of Potassium, I´m currently feeding with GROWBLOOM (BIOMIX) and the SuperVit supplement sample I received as a gift. I will surely flush before harvesting, but for now, I´ll try not to overfeed them. Cheers @Basementganja The rest are doing fine so I am positive about this one too. Surely she´ll recover and shoot up, Northern Light is a robust and resistant plant. Cheers

Look like overdosing on potassium. Check your soil. Check feeding calendar VS plant age. Look up watering techniques. Look up on flushing. Prognosis, don’t do too much and the plant will be fine. 😀😀😀😀

If your other plants are fine within the same space, and live in similar conditions, it's likely a mutant, or picky eater. She might grow through the issue.. With seedlings, you win some, and can lose some.