More than likely aluminum corrosion coupled with wax lube would be MY guess ,without photo can't say with any certainty .
For what it's worth ,I took ALL of MY dies out of their tool holders a few year back , dissembled them and clean everything with acetone and a small bottle brush . Before I ran a small rubber mandrel wrapped with 1500 grit wet&dry on a portable Makita drill driver .
MY old reloading area was in our old garage and as we had German shepherds our garage was open most of the time ,except at night . So even though bench was covered moisture was still present occasionally and thankfully it didn't screw anything up permanently .
I found two of My 9mm & 8mm dies with surface rust ,as it was probably My least used set,so decided after we moved I'd clean them ALL .
After getting most all of MY Dies and their expanders coated I sprayed the remainder with Boeshield T9 problem solved
As ALL have secured lockrings ,simply screwed them back into their perspective tool heads and all is well in MY loading universe
