Hi, glenn2189,
As long as you copied your iMovie library to an external drive that is formatted Mac OS Extended (Journaled) or APFS, you can delete the library from your Mac internal HD. However, before deleting it make sure that the library on your external drive is functioning correctly. You should have your iMovie library backed up on a second external drive as well. That way, if your first external drive fails (and they do) you will have a back up. Note: If you are using a Time Machine backup, your external drive may not be backed up by a Time Machine backup if the external drive is not selected to be backed up in the Time Machine settings.
If you need to reformat your external drive per the above, be aware that reformatting will delete all data on the drive. So before reformatting back up any data, including the iMovie library, that you want to keep. Best to reformat, if you need to, before copying over your iMovie library.
-- Rich