Leafs are fragile. Excess air movement and friction, or any abrasion of the plant to even itself, can result in that. Nothing to sweat, you’re all good. Healthy looking seedling. You’ll be fine, keep up whatever you’re doing.
Cheers
Sometimes insects can invade your space and feed on leaves. I've seen tiny grasshoppers leaves marks like that until I find 'em and pinch 'em. Other than that heat stress can make leaves brittle and they can crack.