According to louie :
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.

Generally speaking, if you leave the flux on, it'll corrode the copper
for a short while until the active ingredient in the flux is "used up".

It'll usually be "used up" _long_ before it actually presents a
leakage risk. The only result being a slightly unsightly joint.

It's essentially a purely cosmetic issue.

If it bothers the OP and the flux is still relatively fresh, hot water
and some sort of degreaser (even liquid dishwashing soap) will remove
the remnants of the flux, any remaining signs of corrosion (discoloured
bits) are not a problem.

If the flux remnants don't come off, it's "used up" (dried out), and
is no longer a problem and corrosion has already stopped.

The only time where you'd do anythimg more is if the copper is
a visual component of the interior decoration. Ie: exposed copper
in upscale bathrooms. Then you worry about polishing and shellac/varnish
to prevent oxidization (if you want it to stay bright).
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.