hi barbara,
ControlCanvas is just an empty container in controlP5 and doesnt have an x and y position. a controlCanvas only knows PApplet as its parent, this is why the mouseX and mouseY dont work (coordinates are shifted) inside e.g. a ControlGroup which i am assuming you are using. using a controlCanvas from within a ControlGroup requires the canvas to know the coordinates of the group, the current parent. these coordinates should then be used to be subtracted from mouseX and mouseY.
maybe the following example does/shows what you are looking for? (square turns red when mouse is outside, changes color randomly when inside).

Code:

import controlP5.*;

ControlP5 controlP5;
ControlGroup l;

void setup() {
  size(400,400);
  frameRate(30);

  controlP5 = new ControlP5(this);
  l = controlP5.addGroup("myGroup",100,40);
  l.addCanvas(new TestCanvas());
}

void draw() {
  background(0);
}


void controlEvent(ControlEvent theEvent) {
  println("got an event from "+theEvent.controller().name());
}


class TestCanvas extends ControlCanvas {
  public void draw(PApplet theApplet) {
    // store mousePosition relative to ControlGroup l's position
    float myMouseX = mouseX - l.position().x;
    float myMouseY = mouseY - l.position().y;
    
    // check mousePosition against myMouseX and myMouseY
    if(myMouseX>0 && myMouseX<100 && myMouseY>0 && myMouseY<100) {
	theApplet.fill(int(random(244)));
    } 
    else {
	theApplet.fill(255,0,0);
    }
    theApplet.rect(0,0,100,100);
  }
}