rust, tarnish
銹 centers on metal degradation: rust forming on iron or steel, and by extension tarnish on other metals. Both meanings share the idea of a metal surface corroding or losing its luster.
銹 combines the metal radical 金 with 秀, which likely provides the sound. The character specifically denotes rust or tarnish on metal.
Metal 金 that has been exposed and become 秀 (a component suggesting something protruding or exposed) is rusted and tarnished. Picture a shiny metal surface with patches of rust breaking through.
For シュ, imagine a rusty shoe made of metal: shoe -> シュ, and the metal shoe is covered in rust.