Green copper pipes, plumber didn't clean up flux after job

[SimonG]

But then I've seen a copper pipe covered in green and pin holed all over.

 

[Basher]

About 10 years ago the National Physical Laboratory published a guide on corrosion in copper pipes. Most of the content is about corrosion on the inside of pipes carrying fresh water (high oxygen content - as opposed to CH systems) which for rare occurrences of particular ph and mineral content, can be very severe.
Interesting read, although it doesn't really help the OP !

https://www.npl.co.uk/special-pages/guides/gpg120_corrosion

 

[Best]

1962 council house with green staining from flux on original copper pipework in loft. Sold in 2012 and the flux still hadn't eaten through the pipes. Let's not over-scare the OP, eh?

Never did try to scare the OP Ric!
But I wouldn’t take any old excuse or lies from anyone responsible for something I am paying for.
Flux on all pipes is a tell tale sign of careless work. We wouldn’t accept a painter to leave paint splattered over floors or windows would we?
I have seen flux thick on cold pipes from the 1960s and only minor corrosion, if any at all. But that was most likely grease based traditional type Fluxite.
However, as am sure you will admit, some fluxes will cause deep and pitting corrosion on copper pipes within months or certainly years if left on pipes and personally I won’t use such fluxes as they damage pipes internally and the solder ironically.
If the OPs plumber forgot to wipe the couple joints, okay it can happen - I have very occasionally forgot to clean a joint & discovered flux on it after joint was cold, but in my experience plumbers rarely clean the joints and that is inexcusable.