Hi, I have same problem. Its uppear when you try to reimport you skeleton with new bones (added in 3d software). And here my decision.
- Create new project whith same hierarhy of folders (from content to you skeleton).
- import in this project new skeleton whith changed or added new bones
- migrate skeleton asset to you primary project. if folder path are same - you got updated skeleton whithout this gray bones.
if you know another speed way, tell pls.