Having lived for many years in the northern states that love to use so much salt that even the road furniture and bridges rot away at scary speeds, I've seen a lot of corrosion issues with brakes.
Probably the worst place for corrosion is the pad backing material, which can tilt the pad relative to the piston, as can corrosion on the corresponding outer face of the piston, which starts once the (typically zinc) coating has abraded away). Corrosion on the pad side of the backing plate can creep under the material and eventually separate it. Fortunately that is not apparent on this pad.
Sliding calipers can also suffer from corrosion that prevents their free movement and result in uneven force being applied to the pads such that one of the pair on each disc can wear much faster than the other. You need to look at ALL the pads to determine if the rates of wear are uneven. Usually it's the pad next to the piston that wears faster once the caliper starts sticking, but sometimes the other one wears faster if the caliper frame is holding it in contact. when the brakes are released.
The interface between the hub and the rotor is another area where corrosion can build up, and can push the disc out of true, usually resulting in a detectable judder on applying the brakes. If you're not doing tire rotations regularly, this can even put so much force on the studs that there'a risk of them breaking.
Onto the rotor itself - the vents between the discs that form the outer faces are spaces between a series of cast webs, and I've seen these corrode to the extent that the the rotor is close to the point of collapsing and the inner disc separating. The corrosion that builds up in this area is undisturbed by much that goes on around it, and can eat away at a lot of material unnoticed, though this usually take several years to get to a dangerous point.
And finally - the braking surfaces themselves. The bands at the inside and outside of the face can accumulate thick layers of rust. The inside of your looks to have a fair amount of rust, but that's because it isn't contacted by the pad, so nothing knocks it off. This isn't harmful. The outer edge also has a fair amount of corrosion, and this can go two ways - it can build up, and abrade away the pad material, as it's substantially rougher than the machined face that remains clean from pad contact (apart from the flash rust that will accumulate every time the vehicle stands, but is wiped of as soon as you use the brakes for the first time). Since the pad is riding over this surface, it can also keep knocking away that rust scale, and leave the disc surface lower than the active track of the pad over the face. Yours seems to have some of one or both of these effects, and there's a band around the outside of the pad that's distinctly different to the rest.
This is probably reducing the effective area of the pad. Will you notice any drop-off in braking performance? Probably not - pressure from the piston will be distributed over the remaining area and braking effort will not change noticeably. If it were my vehicle, I'd leave it for probably another year, there's enough meat left on the pad to not wear to a dangerous level in that time, but I'd be watching it. Eventually the pads and rotors will need replacing at the same time. I've given up on the idea of refinishing rotors with any corrosion issues, even when there's sufficient material left to allow them to be turned flat within the recommended minimum thickness. New ones are cheap enough to not make it worthwhile.