This definitely looks like leaf miner damage to me. Leaf miners on brassica crops are usually flies that lay eggs in the leaves. The larvae hatch and eat the tissue in between the upper and lower surfaces of the leaf. If you just see this one leaf, and your plants are relatively large, then you can remove the leaf and your plants will probably be fine. If the plant is still a small seedling, a few more of these on the plant could do a lot of damage. The best control is parasitoid natural enemies, but to keep the adults off the plants, you can also try a floating row cover tightly sealed along the soil surface.