It is very normal and actually necessary. Rabbits produce 2 types of feces, the harder pelleted feces normally found in a rabbit cage, and the soft, greenish, mucous covered feces. These softer feces are actually called cecotropes. The cecotropes usually won't be found in the cage, as the rabbit normally eats these as they are produced. As gross as it sounds, cecotropes are quite nutritious being high in some vitamins, etc. In fact, the production of cecotropes is a very important and significant part of the digestive sytem function of rabbits.