I wasn't sure what is going on. If lines like this continue to show up, I would guess you have insects burrowing and eating tunnels through the leaf. You can inspect underneath etc. But I wouldn't start freaking out over one line, only if it continues. You may want to do some preventative measures and spray them (near lights out, not during lights on) with something like Safers Concentrate (EndAll) containing insecticidal soap and pyrethrin, but observe first.
Looks like someone set a watering bucket or something on the leaf when watering and it caught on the planter edge.