This is the antirust coating, it is only removed from the pad contact area and not the hat. It does a good job of protection, Infact all brake discs are shipped with some antirust which is cleaned off with citrus degreaser or brake cleaner.
btw - the pad carrier is the big iron piece that holds the pads in place, the little steel shims you get with new pads are called anti rattle clips and are supposed to be renewed as they are actually springy.
The only correct way to do a brake job IMO is to remove the caliper, remove pad carrier, remove pads, remove rattle clips, buff the pad carrier on a wirewheel bench grinder or manual wire brush, after that a dab of silicone grease is applied to the pad carrier ears and the rattle clips are locked in place. install pads and make sure they can slide smoothly all the way in and out (prevents pads from wearing uneven), remove pads to install grease on the pad corners, install pads back.
replace disc (if doing so) - lock the disc, install the pad carrier and pads, remove and clean the slide pins, make sure boots are nice n pliable with no tears, slather on silicone grease to the pins, open brake bleeder, install bleed bottle setup, push piston back, close bleeder install caliper with greased pins and tighten everything to spec.
dont forget the brake wear sensor