Just to help others and perhaps help you make your plugin better, I have found what the problem was:
I was asked to insert this pixel from facebook via email (facebook gives the option to ask someone for help embedding a pixel by entering that person’s email), so I got the email, copy and pasted the pixel code from the email into your plugin and got the error.
Why? because the email contents held hidden characters that didn’t show, but after rendering the code by the browser), hence the “strange” character – you couldn’t see it physically, not in the email and not in the plugin text box.
So how did I solve it? by copy-pasting straight from facebook rather than from the email they sent me.
I think you should be able to filter the text better as the plugin developers, because not always the developer is given access to the facebook (or other) admin panel. There are functions that can filter the text from the user and sanitize it.
hope that helps!