It's totally normal for sweat to get on the cover with moderate use.You can just wipe it with a rag, but the best way to clean it is to put warm soapy water on a cloth and thoroughly clean the surface with it.Other solvents such as isopropyl alcohol, ammonia-based cleaners, and oil-based cleaners work fine too. Do NOT use acetone (nail polish remover), it will 100% damage the cover and/or the device it is applied to.