Langite is a copper mineral formed from the oxidation of copper sulphides.
Obviously, an R1155 has lots of copper wiring in it!
So it may indicate that sometime in its history, someone has identified such corrosion. If this radio has spent a large chunk of its life in close proximity to many lead-acid batteries in a charging room, that would explain it. The dreaded 'green spot' now springs to mind. So I suggest a thorough investigation of as much wiring as is accessible. (Err, this is an R1155
).
Al.