Right, surface doesn’t have a setUndecorated() function, presumably because not every renderer supports it.
There are other approaches in the link posted by GoToLoop. Here is one that’s probably better than the one I posted above:
void settings() {
fullScreen();
}
void setup() {
surface.setSize(500,500);
}
void draw() {
background(0);
ellipse(mouseX, mouseY, 20, 20);
}