Sometimes after I move or route a trace, a small square shows up at the vertex. Some traces have it, some don't.
What does it mean?
They are normally there to indicate a connect point. If you have used the Place Replciate and created a module with partially routed tracks then the connect point would be on that endpoint. It is actually routed and joined but just shows the "join" point. If you didn;t want to see it you can change the size (setup - design parameters - display - connect point size) or disband the group and reroute just past the point.