Besides not knowing what bug took a chunk out of o...

As far as I can tell, the discoloration could be one of two things. Either A, the soil was just a tad hot for the seedling which cause some minor burns but it'll live through it. Or B, water droplets that fall onto the leaves during waterings and once back under the light act as magnifying glasses and causing the burns. A pic (under red light) in your second week of veg shows what seems to be a droplet on a leaf which is why I would suspect this to be the most probable cause. Your new growth seems just fine, nothing to worry about.