Flexible content itself does not contain a unique ID. However, you can easily append one when outputting the flexible content. For example, if outputting your slider wrap it in a container such as:
<div id="slider-container-<?php echo $counter++; ?>" class="slider-wrapper">
Container content...
</div>
Define $counter before your loop and set it to 1 or 0.
If you’re using jQuery you can also set the context for the selector so it does not apply it document-wide: http://api.jquery.com/jquery/#jQuery1
