Please don't take up this daft suggestion to contact TfL.
Rust (at least surface rust) is perfectly normal on steel girders. Once exposed to the air, the iron mixes with oxygen to form iron oxide (rust) which quickly coats the surface of the material. Almost any building using a steel superstructure such as girders is brown with rust in the early stages of construction, almost within a few days of being exposed to air.
Unless the girders are being eaten through by rust (an almost unheard of result, otherwise all of New York would now be lying in piles of rubble) there is really nothing to worry about.