Примерно так:
var balls = [];
//repeat
draw = function() {
if (mouseIsPressed) {
balls.push({x: random(0, 400), y: random(0, 400), vx: random(-10, 10), vy: random(-10, 10), r: 50});
}
background(202, 255, 97);
fill(66, 66, 66);
for (var i = 0; i < balls.length; i++) {
var ball = balls[i];
ellipse(ball.x, ball.y, ball.r, ball.r);
ball.x += ball.vx;
ball.y += ball.vy;
}
};