What happened to this plant?
The leaf spotting looks to be caused by powdery mildew, which doesn't always produce an obvious white/gray coating on plant leaves depending on the host plant and the environmental conditions. This will probably not cause serious harm to the Sedum's health and so do not need treatment. In addition, since pollinators tend to flock to Sedum flowers, fungicide use may be risky as a few are known to affect certain insects. Otherwise, several fungicide products are labeled for use on powdery mildew and might be of use prior to bloom; check the label instructions for use, as most will probably require multiple applications for best effectiveness. Fungicides are preventative and not curative, so afflicted leaves will not "heal."