Mother of pearl paper underneath old wallpaper.anybody know why?

[motherofluvlies]

hi we have just stripped our walls of v old wallpaper and just found a peculiar area covered in very thin silklike mother of pearl type paper.Anybody know what this is?The particular area it is covering looks like it has/had black mould on it but not any of the other walls.We were going to paint i.e. if we ever get the walls in a good enough state.
Should I be concerned?

[motherofluvlies]

Anybody guru DIYERS