I have some catching up to do, don't I? The supposedly last coat of exterior plaster was applied on Friday and they covered it with lightweight clear plastic to keep the rain off while it cured. Saturday morning, it was raining harder than expected, so he kindly showed up to put on heavier tarps held down at the top by pieces of slate and lumber. We then went out to lunch and came back to find this result as the precipitation proceeded to turn to heavy sleet and then snow.
It seems that water running on the surface, when the plaster is not fully cured, pulls the mineral content to the surface, thus rivulets of calcium and other deposits. He's going to try the manufacturer's recommendation of cleaning it with a vinegar mixture. If required, he'll do another coat, but probably not until Spring.
We rejoiced in the possibility that the plaster isn't ruined by going to the neighbors' fun-filled Halloween party dressed as Sue Sylvester and Male Cheerio Cheerleader from the TV show Glee.