White mold (?) in wooden bench's crevices: how to test and clean
I think that dark strip is a metal spacer.  I think the white stuff is white rust



https://www.sharrettsplating.com/blog/corrosion-resistance-of-zinc-plating/

White rust is zinc oxide forming on galvanized iron.
Test the strip with a magnet.  If I am right it is magnetic.   If that is the case there is no way that is mold.  The white stuff can be removed by scrubbing.  Wikipedia recommends oil to prevent white rust. I suspect that furniture polish could probably do double duty and protect the metal from oxidation as well.