Green Marker inside clip (how to remove?)

You hit the M key while the clip was selected. Which created a Marker. If ... without moving the playhead ... you'd hit the M key a second time, the Marker's dialog box would have popped up, where you can name them or make a note in the description area. Set a color, so you can have markers for various purposes or people.

A marker is a ... mark. To tell you or someone else something. A way to store notes.

You can use commands to go to the next or previous marker, or you can open the Markers panel, and even navigate to places on the timeline by selecting a marker. You can read the notes for that marker.

And of course there are commands to delete markers also.

Using the Help menu, get to the Manual and check the section on markers. Very useful tools.

Neil