O-ring abrasion is the cause of this. The deposit is a fine suspension of rubber dust. It was also an issue with the original clutch masters for sure. I believe the Tazu has been castigated for having similar problems. Over the years, the clutch master issue has been a thorny one and after various ventures away from OEM, the preference seems to have cycled back. At least that's what DMGRS says and he should know.
Link:
https://www.dmgrs.co.uk/collections/...5480-stc000110
TC