Thallium
I am like thallium is soft and malleable. Your words can cut through me like a knife. I can be flattened and be forced to conform to a particular shape. As the element has this luster, I have this glow around me that is easily marred because of reacting to other elements in the atmosphere.
As thallium is undetectable because it’s odorless and tasteless, I may not be noticed even if I’m already there. But then again, thallium is poisonous lethal and so I can be.