According to Gary Slusser :
"AJScott" wrote.

That ick comes from electroylisis, which in the plumbing world, happens
when two different types of pipes come into contact with each other, and
the results tend to show up at the joints, as you noticed. Most often
happens when someone uses tin/galvanized hangers to tie the pipes to
wood structure, or lays conduit right over the copper pipe without
sticking a piece of wood shim or cardboard or hunk of foam or whatever
to prevent direct contact between the two un-like pipes, or when someone
replaces a section of copper pipe with galvanized without using a
dilectric union, which is made of plastic, rubber and brass (brass
doesn't react with other metals, which is why brass is used for pipe
fittings).

It can be caused by soldering flux too, which if it is happening at a
fitting, it's much more likely it's flux. Brass does too corrode, just more
slowly.

Galvanic corrosion is always a possibility, but only where dissimilar
metals touch.

On a straight copper fitting, it won't be galvanic.

Many corrosion products of copper are green (eg: chloride salts).
IIRC, copper sulphides/sulphates are blue.

Most of the corrosion products of lead are white - that's why white paint
used to be based on lead ....

Green suggests that it's most likely copper corroding in contact with
something containing chlorine. Ie: salt (road or table) or plumbing flux.

On plumbing, it's probably flux. On automotive wiring, here it's usually
road salt.

White can have a number of different causes, but they're mostly to do
with water on the surface interacting with the solder. Whether it be
pinhole leaks or condensation or whatever.

In particular, condensation on lead-based plumbing solder is probably the
cause of most white powder deposits. But evaporation from a pinhole
leak leaving water hardness "behind" as it evaporates is also a
possibility (in many cases the crud buildup plugs the pinhole leak).

You can see that with objects made out of lead - subjected to long-term
repeated dunkings or condensation, lead picks up a very soft whitish "fuzz".
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.