May be some sort of oxidation on the zinc coated screws. Try simple green and a tooth brush. A polish like flitz would be the next step if simple green alone doesn’t help.

Yup, chalky white rust. Comes off with metal polish and a rag. Buff the head of the screw back and forth on a rag with a drop of polish. (cheap stuff from Warlmart works fine on screw heads and habitrails - "turtle wax chrome polish and rust remover")

Just replace them. Likely the plating is gone and all that polishing will do it turn them black.

For the plastic post buy Duragloss 462 Ultimate Orange Concentrated Cleaner mix it with hot water and let sit for few hours. Will clean the plastic post to like new.

For the screws .... replacing is the easiest and quickest but if you want to use the originals I put them into a drill and then use super fine steel wool and Mothers Chrome polish.