It may be due to an injury such as a cut from a feeder or waterer, especially the metal ones. It could be an abscess or a tumor, but it also may fall off eventually. You may want to put a little iodine on it daily. Look at the picture in this thread, and take a look at the links in post #5: https://www.backyardchickens.com/t/905053/10-wk-old-ba-cockrel-with-bump-on-beak-normal