Solution Eclipse Project name has a red fork, but every file underneath doesn't have a red fork.

It's annoying to have this kind of error because you don't know which file is causing the entire project error. So we can open the error button below and see what the problem is.

If this is because Java compiler level does is not match the version of the installed Java,

So we open the project's properties and have an option for project facets,

Click Open, select Java version on the right to the latest, then apply, should be OK.

It is also possible that the referenced jar package is invalid and can be re-referenced according to which jar deletion is indicated.

Solution Eclipse Project name has a red fork, but every file underneath doesn't have a red fork.