In the show,
it seems that everything mountain men have, beyond food (which they state is from their hydroponic gardens) is very old.
The Mount Weather shelter is shown to have varst warehoused of stuff.
We see mostly the art sections, but I suggest that there is a lot more.
This would also explain the rather unchanged style of the cloths.
If they were making their own, then the fashions would have shifted.
Since they have been just pulling them out of storage,
they are still 20th century fashion.
The white marks are thus likely part of the attempt to make it look aged.
Artificial distressing as discussed in the comments.
I suspect it is suposed to be dust.
You can see the more effective grey dust markes all around it.
It may be that the white marks were suposed to get further treating which got forgotten, or messed up,
ot it may be a plain screw-up:
Costumer: "I thought this would work, but it didn't"
Director: "Well we need to shoot this today, we don't have time or budget to remake these costumes."