Hi gilmorerw,
Thanks for contributing to the Apple Support Communities.
It sounds like you're using the green/blue screen effect in iMovie, but the green is not being removed.
To set up the green screen effect, be sure to follow the steps in the article below for your version of iMovie:
iMovie '11: Use a green-screen or blue-screen effect to superimpose video
iMovie (2013): Use a green-screen or blue-screen effect to superimpose one clip over another
For example, in the latest version of iMovie (2013), it's important that the playhead (that shows which clip is being played) is on top of your green screen footage when you click the "Green/Blue Screen" button.
Sincerely,
Jeremy