this is almost 3 weeks for my plant. it only happens to this leaf and its other identical. this was one of the first leaves to show on this plant . the other leafs look fine
Looks like fert burn to me. Are the leaves crispy? Are you giving them ferts yet or are you using pre-ferted soil? What's the PH of your feed and run-off? Good luck.
I've only dared to use organic MG but from what I hear a bout the rest of their soils, this is most likely the culprit. Those nutes will continue to be released over the next couple months, so transplant out if possible into another soil/medium.
Fox Farm's Ocean Forest is good. Some people think it's a bit expensive. Add some perlite and you're golden