How to recognize movement in webcam?

Since GetPixel returns a Color instance, you've probably noticed that RGB values can be a bit tedious to compare with one another. You're probably also dealing with electronic gain and general camera stuff which will change your pixel values slightly between frames.

Pixel comparisons are a bit simpler with an HSV format, so you might want to try that:

 [Range(0, 1)] public float dh = 0.1f;
 [Range(0, 1)] public float ds = 0.1f;
 [Range(0, 1)] public float dv = 0.1f;
 
 float h, s, v, hp, sp, vp;
 
 void Update()
 {
     Color.RGBToHSV(
         webcamtexture.GetPixel(x,y), 
         out h, 
         out s,
         out v
     );
 
     var hueChanged = $$anonymous$$athf.Abs(h - hp) > dh;
     var satChanged = $$anonymous$$athf.Abs(s - sp) > ds;
     var valChanged = $$anonymous$$athf.Abs(v - vp) > dv;
 
     if (hueChanged || satChanged || valChanged)
         Debug.LogFormat("Pixel {0},{1} changed!");
 
     hp = h;
     sp = s;
     vp = v;
 }