Try increasing the water the plant receives. Yes, Mutabilis can be prone to mildew, but very often the mildew is a result of water stress. If possible, adding more water to help alleviate that water stress may well improve or even "cure" the mildew and is far less damaging than trying to spray with something to correct the issue. I had read in both E. B. LeGrice's and Jack Harkness' books that mildew can be forced by water stress, even on resistant types and in conditions which usually don't favor the fungal issue. I didn't believe it until I did it. Rust can also be forced by too severely water stressing many types.