Green residue on the joints of copper water pipes
The connection is leaking.  What more do you need?
There is water on the outside of the pipe.  It has been leaking for possibly decades and evaporating as quickly as it leaks.  The moisture, along with the solder and possibly flux, has corroded the outside of the pipe, leaving the characteristic green-blue color of dissolved copper.
More importantly, the leakage flow is verrry gradually eroding the sides of the small opening.  The opening is growing, the flow is growing, and eventually, it will start dripping and then pouring on everything below...
I've seen this in a pinhole leak in a copper 90 degree elbow.  The elbow had been installed for at least 30 years in the ceiling over the entrance hall.  The first visible symptom was water dripping from the ceiling light fixture.  When we opened the ceiling, there was an extremely fine mist coming from the pinhole.  All the nails and screws in the joist space were rusted out...