Fecal is an adjective: it means “of or about feces,” the solid waste that leaves the body from the bowel. American English writes fecal; British English writes faecal. Doctors and labs use it in phrases like fecal sample, fecal matter, fecal bacteria. It is a clinical word, not a playground word (there people say poop). Though a short label may call it “the American spelling of faeces,” that name belongs to the noun; fecal is the describing word that sits in front of another noun. Ordinary people often avoid both and say stool if they need a polite word.