That's saliva, right? Right?

What is she holding, anyway?

Maybe konnyaku, or wet sponge.
Staple item in comic when scaring peoples other than ghost costume and fireball. It gave that wet and cold touch sensation when swung on unsuspecting passerby.

With Kogasa, it's invariably konnyaku (aka konjac jelly), as it comes up in her dialogue in UFO.