Unfortunately I cannot think of a solution for this. The content within an IFRAME (e.g. YouTube videos) is outside the control of the website on which IFRAME is being viewed on. So as far as I’m aware, you will not be able to send an instruction between your website and YouTube to ‘stop playing’.
An alternate solution would be to have a slider of YouTube thumb images, and clicking on a thumb opens a popup with a YouTube video playing. An example can be seen here:
https://edgewebpages.com/
(scroll down to the second demo – Video Popups Demo)
Every time a popup is closed the content within the popup is destroyed, so the video stops playing.
There may be a free alternative to SA PRO, I don’t know – you will have to have a look around.
Marking this as closed – nothing further to add here.