Viscous means a sticky material which is thick and resists flowing easily. Viscous is only used to apply to liquids. A Viscous Clot would be a clot which moves a little bit better than a normal clot, which is more a solidified mass of blood. However, clot-type monsters in Vana'diel are simply a subspecies of slimes, so the name is likely just an exaggeration or instance of creative naming.