You need to use this
to select the element which the mouse is on it.
Change Your code to this: jsFiddle Live Demo
$('.photos-list').on('mouseenter', 'li', function(){
$(this).find('.openDiv').slideDown();
});
$('.openDiv').on('mouseleave', function(){
$('.openDiv').slideUp();
});
But i think it's better to use hover() : jsFiddle Live Demo (hover)
$('.photos-list li').hover(function(){
$(this).find('.openDiv').slideDown();
},function(){
$('.openDiv').slideUp();
});