> higher pressure air can hold more water at the same relative humidity

The 787 and A350 cabins have higher relative humidity.

> no. It's not rust,

Citation needed. Corrosion resistance is one of the most advertised benefits of the composite materials in those aircraft[1][2].

1: http://www.lb.boeing.com/commercial/aeromagazine/articles/qt...

2: https://skybrary.aero/sites/default/files/bookshelf/3851.pdf