The Jargon File (version 4.4.7, 29 Dec 2003):

mouse droppings
 n.

    [MS-DOS] Pixels (usually single) that are not properly restored when the
    mouse pointer moves away from a particular location on the screen,
    producing the appearance that the mouse pointer has left droppings behind.
    The major causes for this problem are programs that write to the screen
    memory corresponding to the mouse pointer's current location without hiding
    the mouse pointer first, and mouse drivers that do not quite support the
    graphics mode in use.


The Free On-line Dictionary of Computing (30 December 2018):

mouse droppings

   1.  Pixels (usually
   single) that are not properly restored when the mouse
   pointer moves away from a particular location on the screen,
   producing the appearance that the mouse pointer has left
   droppings behind.  The major causes for this problem are
   MS-DOS programs that write to the screen memory
   corresponding to the mouse pointer's current location without
   hiding the mouse pointer first, and mouse drivers that do not
   quite support the graphics mode in use.

   2.  The client address recorded in a
   web server's log whenever a client connects to a
   site.

   Users may be unaware that their activity is being logged in
   this way but the potential for misuse of the information is
   limited.

   [March 1996 Macworld, p260, Viewpoint article by Larry Irving].

   (1994-12-05)