It turns out they are probably made of a magnesium salt, left behind when a salty brine inside Ceres came up to the surface, and the water evapo/sublimated away, leaving behind the salt.
Another thing that astronomers saw when Dawn finally made it to Ceres was a very big bump. In fact, on the surface of Ceres, it was a mountain. But they didn't know what it was made of or how it was made.
Now they think they do. It's our old pal ice -- this time in the form of an ice volcano.
The article below talks about it. One thing I find interesting -- which I found interesting when I first saw it -- is that it has hardly any craters. That means it's a very young (recently formed) feature.