If it's clone , it's normal for that lower leaf to die off. Hell any plant that crap would eventually die .
If it was mine I wouldn't give it a second thought. The new growth looks fine.
If new growth is getting it, you need to add calmag. sativas always need extra calmag especially if thats an OG. But the picture is a little blurry. Best of luck.
Also, start the calmag on 50% of what the bottle says, 2nd feed you can do 100%