I'm coming to you because I can't find the solution to my problem.
I work on an assembly made up of parts that I have created and parts from a supplier's Step.
I had a green arrow that appeared on the imported parts -> I found the solution by searching on the forum. But elements that have been deleted from Windows Explorer remain grayed out in my build tree. Unable to delete them (right-click or Delete key) Unable to undo deletion -> see image
In this case I have always been able to delete following a change of path for example, I don't get it. Never test with stl files or anything else but there's no reason why it shouldn't work like a part that has changed place.