Coco worms being in their tube for too long or shedding their crown is a sign of stress. I'd watch for things picking on it, shift in parameters, flow issues. These guys have a high mortality rate and can be difficult to keep, basicaly almost needing target fed to give them the amount of food they need, but this can cause parameter issues which will then stress the worm... Quite the double edged sword for such a beautiful worm.
I feel that an established tank that has stabilized and has the ability to support a mandarin is on the same lines as a coco worm. Roughly should be able to give it the food it needs.