Praise be to Allaah.
Firstly: you have to be certain that the walls of the washroom are naajis.
Secondly: If you are certain of that, clothes do not become naajis simply by brushing against something, unless the clothes are wet or the walls are wet, which would make the najaasah (impurity) stick to the clothes.
Thirdly: If you are certain that that has happened, then pray in the naajis clothes. But in this case you have to remove the najaasah itself from the garment, by washing the spot where the impurity is. If you cannot find a clean garment, pray and do not repeat the prayer, because Allaah says (interpretation of the meaning):
“So keep your duty to Allaah and fear Him as much as you can”