This is indeed a type of liverwort. Marchantia polymorpha is a likely possibility, but we do not have specialists in liverwort identification. This isn't something you need to control necessarily. Liverworts generally grow in very moist, shady areas, which would not be an ideal location in which to grow tomatoes. For best results, tomatoes should be planted in full sun (at least six hours of full sun daily) and in loose, well-draining soil that does not stay wet.