1. 'Mucus' is a noun. As in 'During hayfever season, the human nose can exude copious quantities of mucus.'
2. 'Mucous' is an adjective, meaning of mucus, secreting mucus, covered in mucus, or in any way resembling mucus.
3. 'Viscous' is an adjective, meaning thick and sticky, as mucus can be.
4. 'Vicious' is an adjective, meaning spiteful, aggressive, harmful and similar things that are not nice. Unless a mucous discussion is underway, you probably mean this.