If it's happening just in that spot then it could well be windburn. That being said, those sugar leaves look like they're curling down and that shiny look suggests that you might have overdone the nutrients a smidge :)
If you see more damage on other plants that look similar, don't do anything drastic and overcompensate by not feeding them but, you might want to carry out a pour-through test to see if there's any build-up happening.