Looks like some kind of Verbena in the top pic, possibly Verbena bonariensis - these are prone to getting powdery mildew, particularly towards the end of the growing season, and the usual cause is dryness at the roots, and yes, powdery mildew can spread to other susceptible plants. If the soil is dry, its worth watering, but otherwise, yes, the milk remedy will work - 1 part cow's milk to 9 parts water, doesn't matter whether the milk is skimmed or full fat, so long as its cow's. Repeated treatments may be necessary, as with hydrogen peroxide, works as a contact treatment only.
Following your comment, I did a bit more checking - it appears there's more experimentation with milk as a treatment for powdery mildew, so the recipe of 1 part milk to 9 parts water can be varied, using a higher ratio of milk to water, and yes, it seems the treatment is best applied on a bright day, link below
I'd add my personal preference is for using skimmed milk - tends to be fewer problems with being smelly than with higher fat/cream milks.