the acid flux used to clean and solder the pipes. It indicates the connection wasn’t cleaned after it was made. Eventually it could eat through the pipe. It might take 30 years, but cleaning the connection after it is made is always a good idea.
Buy a cheap box of baking soda and steel wool or emery paper. Pour some baking soda on it and clean the connection completely until the copper shines. The flux is now gone and unless there is a pin hole in the connection that should be the last of the problem. The copper will oxidize again but copper oxide on the surface is a protector and totally normal. The green fuzzy stuff isn’t.
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