the marks left behind by some kind of sucking/piercing insect. Thrips and spider mites can be very tiny and difficult to spot. Try shaking the leaves over a piece of paper, see if you see any tiny dots.
In the meantime, your baby tomato plant looks a bit spindly and stretched. It may not be getting enough light and/or nutrients. I can't see in the picture what kind of lighting you have, but it needs to be right down over your plant(s). Being spindly will make them more attractive and vulnerable to whatever insect pests are around.
You want them looking more like this:
short, stocky stems, lots of leaves, not much space between leaf nodes, darker green.