I'm not 100% sure what it is but it will turn dark if you put it in water and set it in the sun. It will also totally dissolve in HCL. I think it is copper I chloride from the copper being attacked by the solution. A barren solution will rapidly dissolve copper. I am still learning so this is just my guess as to what is happening, I'm sure someone here knows for sure. :|
Here is a link to my experience with it. http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=10507&p=101628&hilit=white+copper#p101628
Here is a link to my experience with it. http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=10507&p=101628&hilit=white+copper#p101628