Removing Non-Photo Item in Photos library

As Photos is showing "items" you should remove them. See this user Tip: https://discussions.apple.com/docs/DOC-250002727

As OT recommended, I would use a smart album to search for problematic items.

You may want to search for all items, that are not in a safe, supported format. If most save items in your library are in the format JPEG or or HEIC, search with rules

• Match All
• Photo is not JPEG
• Photo is not HEIC

Add filename extension typical for your library.

I can find problematic items in my library by excluding these filename extensions:

In my libraries the problematic items are usually audio files or PDF files.

Try finding them in the library with Photos using a smart album like this:

Although I'm surprised you even have them listed in the library as Photos is not capable of importing that type of file, only PDF files.

If you do find them try moving them to the Recently Deleted album with the key combination of Command + Delete.

It sounds that those files are by accident just in the ~/Pictures folder, NOT in the Photos library that by default is usually saved there.

You didn't drill into the actual Photos library, didn't you (it can be opened via right/ctrl-click, but it is NOT a good idea to manually mess there unless you know what you are doing)?

But when I use Finder to look in the Users/Pictures folder I do find 6 items that aren't picture or photo file types:

The Users/Pictures folder is not the Photos library. Items in the Pictures Folder are not available to Photos unless they have been imported to the Library. Photos will not import these files. They are quite independent of the Photos Library.