The term 'covered with sludge' can be described as 'MUCKY' in five letters. Sludge is a type of waste material that is created from industrial processes, such as wastewater treatment, and is often made up of a combination of water, solids, and other contaminants. It is usually thick and viscous, and can contain hazardous materials. When something is covered with sludge, it means that it has been completely coated in this waste material.