There are a number of issues that can cause spotting on the leaves of sedum but this combination sounds like your sedum plant is being affected by the fungal disease powdery mildew. It looks different on the succulent leaves of a sedum than it does on most other plants and creates pale, scabby lesions where it penetrates the leaf's waxy surface.

Once powdery mildew occurs, fungicides are no longer effective in remedying the problem. But it is unlikely to kill the plant and there are things that you can do to prevent it from reoccurring next year. Prevention through careful garden practice is the best approach. 

Powdery mildew occurs when fungal spores sit on the leaves in dampness long enough to germinate. So some of the prevention is removing fungal sources from the area and the other part is keeping any fungal spores that do remain or drift in from germinating. Fungal spores germinate in prolonged damp conditions so increasing sun and air circulation help a lot in prevention. Here are the things that you can do.

• Reduce crowding around your plants. Even in full sun, overcrowding, especially as plants become bushy in late summer, will lead to poor air circulation and will create problems with the humidity level around a susceptible plant. In these conditions, leaves will stay wet longer allowing fungal spores time to germinate. Do not allow surrounding trees and shrubs to hold a humid layer of still air around your garden. Thin overhead branches as well as the garden bed.
• During the winter the fungus survives on plant debris, so it is important to clean up all the leaves and other fallen plant material around infected plants. If there is organic mulch around the plant, remove that too.
• Only water your sedum from a ground level source. Sprinklers that wet the leaves can help spores to germinate.
• Using high nitrogen fertilizers (the first number of three on fertilizer labels) promotes weak, leafy growth that fights off infection less well; use a balanced, organic fertilizer with low  numbers and apply only once in the spring to give perennials a boost for the season. Don't let high-nitrogen fertilizer applied to grass stray into the garden bed.

Courtesy of NYBG Plant Information Service