That looks like frass, which is another word for insect feces. I would recommend you take a closer look at your tomatoes to look for caterpillars that might be chewing on the leaves. The tomato hornworm and/or tobacco hornworm generate feces that are very similar to those in your photos. However, there are many different caterpillars that could be doing it. If you don't find any during the day, try looking at the plants at night with a flashlight. Slugs, cutworms, and other insects come out at night to feed on our garden plants.