Looks like one, though I am not sure. Is it possible to take a picture of the tail tip under light so we can see better? Is the tip dry-ish and feels rotten? If it is might have to take it to a vet to make sure it's not spreading.
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
For more info, please see the link:
https://reptile.guide/bearded-dragon-tail-rot/
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