I always thought that was intentional, like little pieces of dust floating around to make things feel more alive.
Viewing post in Our Life: Beginnings & Always comments
From what I can tell it wasn't there until after a scene with the fireflies and moves in the same patterns so I figured there was a code issue. I think dust would float down rather than moving quickly and erratically. But I could be wrong. One of my games (also made in Ren'Py ) uses something similar for insects and snow so I may be overthinking it.