Toxa was on the right track, the issue that cfogelberg raised is valid, but to avoid that issue you can do the following:
forever -c "nodemon --exitcrash" app.js
this makes sure nodemon
actually exits (rather than giving you the "app crashed" message) and then forever
picks it up again.
In forever --help
this -c
specifies a command to run otherwise it defaults node. Without -c results in the error that is mention in the comments to this answer.