It looks like its gonna shed but here is some info to look for or cross reference to see if it is tail rot
Causes of tail rot are:
1. Injuries/trauma - caused by falls, or by any other bearded dragon.
2. Malnourished - make sure he has a balanced diet and he gets the nourishment he needs
How to identify tail rot:
1. Dried out - Decaying tail won't be able to retain moisture, check if it's dried out completely.
2. Very thin, brittle - Decaying tail won't be able to process nourishment, causing it to become more thin and brittle.
What to do after finding out it has a tail rot:1. Go to a vet - very recommended
Karrie