What could this be?

Hi there! It looks like you got some miners, digging into your leaves...I'm more than used to them! First, always remove affected leaves if it's possible (3-4 a day max). Secondly, find some neem oil, mix a teaspoon of neem oil with 1L of hot water, let the mix rest and get tepid, then use it as a foliar spray onto your plants at night. These little bugs hate neem oil, you'll see the invaders dying one by one ;-) Finally, you can try some products to protect your leaves against such invasions, I use BioBizz Leafcoat, which is basically a shield for your leaves. Hope this helps, keep us up-to-date and happy growing!