More like mildew. Sedums are a bit iffy about fungal sprays, so probably not a good idea to use one. Try the 9 parts water to 1 part full cream milk spray on part of the plant first.
Otherwise find a clean bit of the plant and use it as a cutting.

It looks like mildew. I'd cut it back to the ground and see if it regrows. Succulents like gritty soil and plenty of air circulation so it may be a good idea to move it. It isn't in the right place right next to that other plant.

Thanks both. I think I will cut it right down to the ground, then cut away some of that Bowles's Mauve plant that's to the left of it. I never use chemical sprays of any sort so may try the milk/water mix.