wash
沐 centers on washing or cleansing with water, often implying a thorough rinse or bath.
沐 combines 氵 (water) with 木 (tree), likely suggesting washing with water poured from a wooden vessel or washing under a tree. The exact historical development is uncertain.
Water 氵 pours over a tree 木, washing its leaves clean. Picture a refreshing rain rinsing the tree: 沐 means wash.
For モク, imagine washing a mock-up tree made of plastic: mock -> モク, and you rinse it with water.