What you are seeing is probably insect damage from the vegetable leafminer (Liriomyza sativae) or cabbage leafminer (Liriomyza brassicae). These two insects in the fly order Diptera are usually greenhouse pests and do sometimes appear together. They lay eggs in vegetable plants, including tomato, cucumber, lettuce, melons, beans, pepper, cabbage, and other cole crops, and in many flowers. If you bought other plants and had them inside together, the insects probably came in on the greenhouse plants.
The eggs are laid by puncturing a hole in the leaf. This is probably what you are seeing on its underside. The eggs hatch into larva, which then eat and tunnel in the interior of the leaf in meandering lines. The leaf surface over these lines does die after a day or two. This damage is not serious unless there are more than two or three mines per leaf.
When the larva is ready to pupate (form a cocoon) it cuts a slit in the leaf and pupates on the leaf or in the soil. Therefore, it is important to clean up any damaged plant debris and burn it or put it out with the landfill trash. This eliminates any unhatched insects still inside or pupating on the leaves.
Control using insecticides is challenging. Contact your local Extension office for recommendations.
