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?
yeah it reacts with iron; normally all copper is to be hung with either copper plated or epoxy coated materials, and taped/covered wherever it may come in contact with iron/concrete.