Is it possible to move the mouse by one pixel on a retina display?
No - the mouse is aligned on the "fake" size of the screen and not the true screen pixel grid in most cases.

The concept of a physical pixel and a display pixel is a distinct one on retina displays since the hardware reports movements on a resolution-independant scale.

Furthermore, there appear to be some subtle bugs in the code side of things so even if the developer wanted to determine the next pixel's color it appears the routine that reports mouse position doesn't count physical pixels on this model screen.

There is a nice, very technical article by one of the programmers for Xscope - Retina for Masochists/ that explains a bit how the resolution that the software is told to draw gets translated into the actual on screen pixels which is fixed.

As an aside - the Xscope program is free to try and puts the digital color meter to shame on many aspects of determining color and much, much more.