Green residue on soap dispensers...

[CJJ]

Mrs. CJJ* wants to know. We have some decorative soap pump-jars (ceramic with a chromed metal top). Whenever she replaces the soap in these, she finds a layer of green residue crusted onto the rim of the metal top (the soap is orange in color). She wants to know: what is this, and perhaps a little of the chemistry behind it (I always hated chemistry in school )…

[cornflakes]

IANA metallurgist, but heres my guess…

Ornamental chrome plating is done by first plating an object with copper, then nickel and then chrome. My guess is that the nickel or copper has oxidised in the presence of a base (i.e., the soap) and the green stuff is nickel or copper oxide.

[Squink]

@cornflakes

the nickel or copper has oxidised in the presence of a base (i.e., the soap) and the green stuff is nickel or copper oxide.

That was also my first guess.
As a second guess, there’s also some funny colors that can develop in soap mixtures containing guar gum:

Addition of the sodium hydroxide to the suspension results in an elevation of the temperature of the system. Surprisingly, addition of the sodium hydroxide to the suspension is also accompanied by a distinct change in color and physical property of the suspension. The milky white color of the guar gum suspension is transformed into a decided green shade. The viscosity of the system increases markedly. At the end point of addition of the sodium hydroxide, and completion of the formation of the guar gum-coconut fatty acid-sodium hydroxide complex, the system has the color and consistency of pea soup.

Thus, nickel, copper or chromium may not be needed to get a color reaction here.