I assume you mean in mammalian lungs, where the mucus is important in helping gaseous exchange. This exchange is controlled by a membrane protein, which regulates Chloride efflux (leakage) from lung epithelial cells. Its main effect is to make the mucus fluid, so your answer is probably water. This fails in people with Cystic Fibrosis, where not enough water leaks into the mucus. This causes sticky mucus and infections. It is due to a defect in a gene which causes expression of a defective protein.