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It is likely to be rust from a wet rotor transferring to the pad and dripping down on the rack.
Btw, the rust on the hub is horrific. Wheel hubs should be spotlessly clean and receive a thin coat of high temp grease to prevent rust from setting back in (I like either lithium grease or the permatex silicon-ceramic orange stuff, gray anti-seize goes on too thick and dries out too fast). Rust scales on a hub almost always put you out of spec for lateral runout on the rotor and are a frequent cause of wheel tightening issues.
But you have more pressing issues: one of the piston dust boots visible in your pic seems torn, and the friction material on the pad turning white is a sign the pad is being massively overheated, I usually see pads that color when I service neglected brakes that are totally seized. The fingers on the outer side of the caliper are also starting to change color, there is way too much heat being put into these brakes.
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