Material is starting to transfer from the serpentine belt to the tensioner pulley.
I don't know how old the belt is, but we've had the van for 26 months and 34,000 km (c. 21,000 miles), so it's got at least that on it.
There's no noise (screeching or chirping) from the belt. There are no obvious cracks.
Change it or continue to run it? Thanks!