Thank you for including an image with your inquiry.  Caterpillars are eating the lettuce and, at the same time, are excreting their droppings – the small dark mud-like bits.  Management at this stage requires a bit of attention but can be rather simple with regular monitoring trips to your lettuce patch every day or two to check the leaves, both tops and bottoms, and then remove any caterpillars you find. Look closely because some are nearly the same color as the leaves.) Dispatch the pests on the spot – either drop into soapy water or cut in half. 

Next time you plant, whether that’s by seed or transplant, loosely blouse non-woven row cover over the patch, and then secure the edges to ensure no moths can enter. (Don’t cover the plants now because you’ll provide a safe refuge for the caterpillars already present.) Bacillus thuringiensis (Bt) is an organic remedy but is effective only if the caterpillars are young (half-grown or less). See http://www.ipm.ucdavis.edu/PMG/GARDEN/VEGES/PESTS/loopers.html