Hey, how's it going?
From the photos, it's a bit hard to tell what it is.
It also seems like a recent problem, which is probably why it's not clear in the photo.
But looking at the issue, at first glance, it could be a nitrogen deficiency or another nutrient.
I don't want to suggest something that might not be the solution, so I recommend you keep an eye on the growth of this problem.
If you have any questions, don't hesitate to ask. Cheers and good luck with your grow!