I have been working with photoshop for many years, and currently using PSE 2020 on Mac Mojave. Soon to move to Big Sur on Mac Mini M1. These files do open in editor, but I have many in my catalog with this symbol. It is a "not" symbol, but "not" to what ? I feel that I should know this. Thanks.
Those photos do not have any keyword search tags associated to them. That photo is not incuded in any search criteria (such as "vacation / mountains / cascade." You get 5 bonus points (they have zero value because I'm the only one that gives them) for posting an actual screenshot, and not a blurry photo of your comptuer screen taken with a phone.
Just to add to Glenn's response, you have currently filtered your catalog with either a search term or a keyword tag. Your screenshot photo shows that it is in a stack/version set (icon in the top right). Your filter has found a matching file within the stack which is why the stack has been expanded to display it. But your screenshot photo does not meet the search filter and the "not" icon shows that it does not. Once you clear the filter, the icon will go away.