Hello RocketReducer62,
Thank you for posting in Apple Support Communities! It's my understanding that you are experiencing issues when using iTunes visualizer. It's always fun to see a colorful light show when listening to your music! I found some information that could help you troubleshoot an issue when using iTunes visualizer. Check it out below.
Turn on visual effects in iTunes on Mac - Apple Support
As mentioned in the above article, if the music you are playing includes a video, the visualizer shows the video by default and could cause some interruption such as what you are experiencing. Check into your settings as suggested in that article.
If the issue remains, and continues even after restarting your Mac and iTunes, I would recommend making sure your Mac is updated to the latest compatible macOS: How to update the software on your Mac - Apple Support.
Let me know if you are still having issues with this after reviewing the above mentioned articles.
Take care!