We hired a plumber to pipe the entire house with new copper pipes a
couple of months back. In the last weekend, I found that there are
quite large areas of green rusts around the copper fitting/connector
sections. The floors and walls are still open, so we can still access
all pipes.

I did not observe the installation so do not know exactly how they
did it. I'd like to avoid repiping the entire house, and want to
see if there is a way to clean up the rust and apply proper sealant
to those areas. My hope is that this clean up will let the pipe last
as long as those laid properly.

Would appreciate suggestions on ways to do this.

An expensive lesson learned, never hire a plumber ONLY based
on price!