Copper stain
Green on copper is a breadcrumb, not a verdict.
Oxidation, flux residue, and actual seepage can all mark a line. Occupants who wipe it off every week may be wiping a leak that never gets logged.
Photograph it dry and after a paper-towel test.
A towel that picks up moisture is different from a dry stain.
Do not sand the pipe ‘to clean it’.
You can thin a wall that is already pitting.
One green mark plus a ceiling stain elsewhere still needs a path.
The stain is local. The damage may not be.
Questions people ask
Is green always a pinhole about to open?
No. It can be old flux or surface oxidation. Moisture on the towel changes the urgency.
Should I paint the pipe?
No. Paint hides the next change.