thats the "crown ring" the worm its self will regrow in about two weeks. Now it may have thrown the crown due to being picked on, water moved to strong, water chem not right. Check out water parms and movment and make sure its just a "thing" it did. one of mine of six, threw his crown twice in like a year period, now he don't close up unless some one is really p-oing him off. Then in for a second or two and right back out. HTH