You must have typed a weird character that happens to look normal within the " xpos.length-1" and the " ypos.length-1".
Also, you have an extra capital "p" on line 8.

Below is the working code:

1. int[] xpos = new int[50];
2. int[] ypos = new int[50];
3. void setup() {
4.   size(200, 200);
5.   smooth();
6.   for (int i = 0; i < xpos.length; i++) {
7.     xpos[i] = 0;
8.     ypos[i] = 0;
9.   }
10. }
11. void draw() {
12.   background(255);
13.   for (int i = 0; i < xpos.length-1; i++) {
14.     xpos[i] = xpos[i + 1];
15.     ypos[i] = ypos[i + 1];
16.   }
17.   xpos[xpos.length-1] = mouseX;
18.   ypos[ypos.length-1] = mouseY;
19.   for (int i = 0; i < xpos.length; i++) {
20.     noStroke();
21.     fill(255-i*5);
22.     ellipse(xpos[i], ypos[i], i, i);
23.   }
24. }

Thanks, had to retype line 17 and 18.