Condition of the wood behind the board

[joppz]

Hello!
I've spent the day tearing out the board from a small toilet that smells musty. We were thinking of replacing all the surfaces.
Behind one wall, I found this piece of wood, that is, in the wall, behind the board.
Is this mold? The wood is not soft/spongy but rather nice and hard. It looks more like paint in texture.

 

[Jonatan79]

Probably old paint or similar, not mold.

 

[Bo arnold]

Probably an old floorboard had a carpet made of paper, the underside of which was tarred and stained the floor. Nothing dangerous, have seen a lot of this in old houses.