I live in a condo (40ish yo) and recently a neighbor in an adjacent building had a leak and the board suggested that everyone have a plumber come inspect their units. I just inspected and found various levels of corrosion at most joints with the included photo the worst looking of the bunch.
How concerning does this appear?
Since it’s blue and around the joints, I’m thinking it’s the flux used when these pipes were put in. Most likely with a bit of scrubbing with a metal brush and polish with some steel wool it’ll come right off. I doubt it’s actually going into the surface.
But let a professional look at it to be sure.