Why do you mean by "act as a mask", and how the current behavior doesn't do what you want? Currently, you draw a rectangle of random gray color at the click position if it is in the top part.

Maybe something like that:

1. float x, y, ax, ay, bx, by;
2. int stRad;
3. PGraphics waveForm;
4. int maskHeight = 200;
5. 
   
6. void setup() {
7.   size(800, 800);
8.   smooth();
9.   frameRate(60);
10.   stroke(250);
11.   background(0);
12.   stRad=100;
13.   waveForm  = createGraphics(width, maskHeight, P2D);
14. }
15. 
    
16. void draw() {
17.   if (frameCount < width) {
18.     int cx = width/2;
19.     int cy = height/2;
20.     float r = random(50, 100)+random(1, 50);
21.     float buffCircle = TWO_PI/240.0;
22.     float buffAngle=buffCircle*frameCount;
23.     float bbuffAngle=buffCircle*(frameCount+1);
24.     float abuffAngle=buffCircle*(frameCount-1);
25.     if (buffAngle <= TWO_PI) {
26.       ax = cx + stRad * cos(buffAngle);
27.       ay = cy + stRad * sin(buffAngle);
28.       x = cx + r * cos(abuffAngle);
29.       y = cy + r * sin(abuffAngle);
30.       bx = cx + stRad * cos(bbuffAngle);
31.       by = cy + stRad * sin(bbuffAngle);
32.       beginShape(LINES);
33.       vertex(ax, ay);
34.       vertex(x, y);
35.       vertex(x, y);
36.       vertex(bx, by);
37.       endShape();
38.     }
39.     float currentHor= (3.33)*frameCount;
40.     float acurrentHor= (3.33)*(frameCount+1);
41.     float bcurrentHor= 3.33*(frameCount-1);
42.     if (currentHor<=800) {
43.       waveForm.beginDraw();
44.       waveForm.stroke(250);
45.       waveForm.beginShape(LINES);
46.       waveForm.vertex(bcurrentHor, stRad);
47.       waveForm.vertex(currentHor, r);
48.       waveForm.vertex(currentHor, r);
49.       waveForm.vertex(acurrentHor, stRad);
50.       waveForm.endShape();
51.       waveForm.endDraw();
52.       image(waveForm, 0, 0);
53.     }
54.   }
55. }
56. 
    
57. void mousePressed() {
58.   if (mouseY <= maskHeight) {
59.     fill(0);
60.     rect(0, 0, width, maskHeight);
61.     copy(waveForm, 0, 0, mouseX, maskHeight, 0, 0, mouseX, maskHeight);
62.   }
63. }