That my friend, is clutch dust, and a small portion of your ORings that wear in Master and Slave cylinder. Mostly clutch dust. Kinda like asbestos or brake pad dust. Your clutch swishes against the flywheel everytime you shift or start off, rubbing the clutch plate surface off as dust. Then it gets on the shaft where the ORing of the CSC rubs up and down as you press/release the clutch pedal. Search for "Ranger Protocol" he'll explain it better than I can.
I would say based on your symptoms your Master Cylinder has failed.
https://www.youtube.com/watch?v=gBbsy0LjUvE