When copper corrodes normally, it forms a green "skin" that protects it
from further corrosion. The others are correct that you can just clean
off the excess flux and polish the copper with some emery cloth. It's
not really necessary, and shouldn't affect the life of the joint so you
can put away the worries of repiping the entire house in 5 years (of
course, if the plumber was sloppy enough to not wipe down his joints
afterward, what else has he missed?).