Is semen considered pure even if it touches the body and clothes, and does it render a sleeping person's hair impure if it touches dry semen spread on the bed and pillow?

Semen is pure. Whatever it touches from the body, clothes, or furniture does not become impure. The fundamental principle regarding things is purity, and one does not deviate from this principle except with certainty. Doubt does not render a place or what it touches impure.