Two things have happened here.
Age cracking of the neck…due to the fact that the US did not neck anneal brass until the mid 1920s…the Forming of the neck and shoulder was not followed by the typical anneal which colours the neck and shoulder as seen on much military ammo from the 1930s onwards.
Moisture from the atmosphere as well as moisture produced by the deterioration of the Powder ( along with Nitric acid formed by the breakdown of Nitrocellulose, ) has caused the blue-green Nitrate/Oxide corrosion of the brass, from the inside out. An analysis of the “ooze” will show all these components ( a good High School Science Lab experiment). The Bullet base (lead) will also be corroded into White Lead Oxide and Nitrate ( coloured by the Copper compounds)
The only solution ( no pun intended) is to pull the bullet, dump the powder, wash the case out with warm vinegar followed by hot water, dry, and re-assemble.
Sadly, all or nearly all of our precious cartridges from before WW II, ( and some after) will suffer from this “Green Cancer” sooner or later.