This question pops up about once every 1.5 years. Back in 2017 it was my turn to ask.
What is this?
Mine too is on the seat stay and you have to get your fingers dirty to use it. Seems the like lower on the stay would be better so you can snag it while dropping the wheel. Or as others have theirs, on the chain stay. I've never used one though.
I guess I could remove it and save some weight! <grin>