What's this?

[Sailman]

Few months back I finished up two twenty inch sections for a CCVM column. Well today I got around to doing the vinegar run. After I shut down I washed everything in warm water and then run a rag through the column. What could this be on the rag , is it the natural patina? The column was made with new copper. I only noticed this when I cleaned one section it did not come out of both. I won't be too concerned about it unless I get the same color when I clean up after my sacrificial run.