Idk if this will help. The only time I ever seen white crap where the bullet meets the case was rummaging through my dad's old very fucking old 38 s&w ammo. When I saw it, I asked how old it was and iirc it was bout 30 years old. He was never into firearms as a sport, prepping, sorta thing. He was a LEO (retired now) so he was required to have firearms.
Anyways, upon inspection I think it was exposed lead or something. I mean I have soft points (223,) 38 spl sjhp/ round nose etc. and all but I keep them in ammo boxes with silcia packs and they dont have that. Some of My 38 spl is prob bout 9 years old and still nice and gray. He didnt keep it in ammo boxes. It was in a leather (cowboy style) pouch. Like the kine you see in cowboy movies on a belt but this pouch could only hold 6 rounds. I can only assume exposed lead cause it was at the case mouth and the white shit literally ran up to the tip of the bullet and on top the pouch. Or vice versa. Of course I turned it into the range officers at kokohead many many moons ago. To expose of it properly.
So I think it could be a mixture of things, possible exposed lead, chemicals on the lead, a blood red moon and the changing of the earth's solstice. Lol. Ok seriously, lead and sulfur. Isnt there sulfur in primers or powder?
Oh and btw it was pitted under the white crap. I wasnt gonna let him keep it nor did we have a pistol to shoot it with so I dunked them in water and scrubbed them. Others that looked alot better I rubbed oil on the brass to clean them. This was just me being curious.
Sorry I don't recall the maker.
I was taught, if it ain't pitted... Shoot it.