The fan bearings are shot.
I speculate that the substance your observing there is copper alloys (brass) or copper, oxidised some, and possibly even other metals like the shaft metals or other bearing metals, as it falls apart more inside and grinds and even more stuff flys off.
Some fans are all "sleeve" bearing style, like many car alternators, and water pumps. A sleeve bearing uses a smooth hard shaft and softer sheath that the shaft turns in, the softer metal sheath is usually alloys of coppers (brass). (or just looks coperish because I do not run elemental analisis :-)
A sleeve bearing fan should be lubricated, and some DO have various "permanent" classed, but semipermanent reality lubrication systems, like a felt that might hold oils that move via capillary flow styles. so It is also possible that some of the contents you might observe on a motor with failings this badly would have parts of the lubrications system in it, or even particals of lubrication itself that might use PTFEs or Teflon and other lubrication "enhancer" things.
If via heat (high friction) and vibrations the bearing or sleeve is no longer being held in place, it could be spinning , and grinding stuff out of the mount even, like hard plastics. (I donno, just blabing)
Get out of it.
Try and get dual ball fans whenever possible, after 15+ years of experience with real ball bearing fans, I can confirm they work better, are quieter, and last longer.
Sleeve bearing fans (and other sleeve bearing motors) have a specific limited lifetime, and generally would want to be replaced as soon as they start making noise, lubricating them late or trying to save them is a waste of effort.
There are also "ball bearing" fans that have sleeve bearings in them (1ball) the ball bearing is in the most nesssisary location where pressure is high, and the sleeve is hanging out on the far end of the shaft where there is not as much pressure, just the tail of the shaft so to speak.
There are also labeled Dual ball bearing fans you take apart and it was just fraud. I would also assume that various incorrect substitutions for the soft metals might exist, which could be way cheaper and a total disaster.
It is also entirely possible for a dual ball to lose a few balls :-) to come apart, to cause some of the same noises , to turn into a metal grinding machine, and they are also supposed to be lubricated, and have a specific lifetime also, just much longer and generally were more reliable.
The majority of fans that I have acquired in a product, video cards, cases, towers, Cpu coolers, have used the cheap sleeve bearing fans, I now replace them immediately , without even thinking about it anymore.