When you're sharing your entire screen, and the screen that you are sharing is visible while on live video you get an effect similar to two mirrors reflecting each other. It creates what is known as an "infinite reflection effect" that looks like this:
This happens with any software that lets you see the screen share while you share your entire screen, even with Hopin.
Don't worry - it's not a bug and nothing's broken. 😀
When you switch away from the Hopin video area, your audience will just see your presentation or webpage that's being shared. For example, if you then open a web browser window this is what will show up instead:
Another way to get around the issue - if you have two monitors - is to screen share on one of them and keep your Hopin page on the other.