Вариант)
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Пример</title>
</head>
<body>
<div class="div" onmousemove="bol(event);"><div class="boll"></div></div>
<script>
var boll = document.querySelector('.boll'), html = document.documentElement;
function bol(e){
boll.style.left = e.pageX - html.scrollLeft +'px';
boll.style.top = e.pageY - html.scrollTop +'px';
}
</script>
<style>
.div{
width: 200px;
height: 200px;
background: #333;
position: relative;
overflow: hidden;
}
.boll{
width: 10px;
height: 10px;
border-radius: 50%;
position: fixed;
background: #fff;
transition: all 0.2s;
}
</style>
</body>
</html>