It's the "Old Man" slot.
https://github.com/kwsch/PKHeX/blob/0c0651f7192cda4f344507b6ca19c3d64c03ff63/PKHeX.Core/Editing/Saves/Slots/Extensions.cs#L115
Daycare slots are faded, to indicate that the data there has been retrieved. The game doesn't clear the data, only marks it as inactive.
GTS: The "inactive" flag is undocumented, so it won't fade if unused.
These slots just show extra data, if you're looking for it.