They will drop their crowns when stressed or other reasons. Think of this part as their wig.lol that they feed with. They can grow this part back a few times. But it does feed with this so now that it is gone you will need to help it feed. Take a ML dropper and fill with some type of coral food or small other and spray dirrectly into its tube to assist it with feeding. could take a month or so.
Also you would get better attention to this question in the invert forum rather than the fish dissccussion