You could spray the substance with water from the garden hose. If you think it is insect droppings, carefully inspect the lettuce for any signs of insects/caterpillars and hand pick them off the plants. Cabbage looper and corn earworm are chewing pests of lettuce. Aphids, leaf miners, and flea beetles also damage lettuce. You didn't mention if there is any damage to the leaves of your lettuce. If there is, and you can write back with those specifics, we can better help identify what type of insect may be present.
If you have a hand lens or magnifying glass, inspect the substance and see if it is moving. It could be some kind of larvae. Make sure to look at both surfaces of the leaf, and scratch around in the soil at the base of the plants.
