It's tempting to conclude that the alloy is pure or nearly pure lead. But wouldn't pure lead have difficulty filling out the bullet molds? I thought that almost all alloys needed some tin in the mix to assure complete filling of the mold cavities.
At least for me, when I get "slushy" stuff on top and it won't go away with fluxing, the melt will also incompletely fill the mold cavities. When I add more tin, both problems tend to go away. I'll still have a very thin layer of "slush" that still won't go away with fluxing, but it's a very thin layer, and the mold cavities fill out completely. That's just what I'VE observed.