Green Residue on Plaster Under Wallpaper?

We've taken off many layers of old and difficult to remove wallpaper from our 100 year old house. One room has some kind of green residue on the plaster, under the last layer of wallpaper. We've washed most of it off, and have noticed that the rags, if left sitting, just get eaten up by the green stuff.

We can't seem to get it all off the wall. What is it, and what should we do?

By SJ