Thank you for reaching out.

The sedum seems to be suffering from fungal disease, likely powdery mildew. Powdery mildew is a common fungal disease that affects many plants like phlox, hydrangeas etc.. It often develops late in the season. High humidity, dry weather, cooler night temperatures and dew often contribute to its severity.

Some information about powdery mildew.

https://extension.umn.edu/plant-diseases/powdery-mildew-flower-garden

https://www.umass.edu/agriculture-food-environment/greenhouse-floriculture/fact-sheets/powdery-mildew-diseases-of-ornamental-plants

Cut back the infected plants, and dispose of the foliage. Good cultural practices are important for managing powdery mildew, especially late in the season. Clean up the area where the infected plants are, rake up and discard infected leaves to reduce overwintering of any pathogen material. Watering should be done in the day so plants can dry before night time. Avoid overhead watering, thin surrounding vegetation to reduce humidity around the plant and increase air flow.

Spraying fungicides now will have little impact as fungicides should be used early in the season before symptoms appear on the plants. Even if it was used in the early season, it may pose a serious threat to pollinators so it is usually not recommended.

Hope this helps.