I'd rule out light burn, because it's on the bottom leaves and the top ones look good. Doesn't really look like nute burn to me, more like deficiency? but not certain, I'll let someone with more grow experience than me answer that one.
Be careful not to vedge that Blue dream for to long, Humbolts version is supposed to double in size during the stretch, and can get to 3 meters outdoors! The freebie Blue Dream I am growing is making me believe it, kinda a little scared of her. Recovered from early problems, and looks like a solid strain!!