I wouldn't jump immediately to internal parasites since there are other reasons for white feces such as constipation from low quality foods, other food related irritants such as bloodworms, and even water conditions leading to stress. What your seeing is the intestinal lining which is the result of attempting to pass something, so I wouldn't be to concerned unless you start seeing this continuously and other symptoms such as reclusive behavior, refusing to eat, and sunken stomachs.