I agree with Karrie
Believe it's tail shed..
Tail rot is when the tail blackens & attempts to fall off almost like a crested gecko except bearded dragons tails don't naturally come off like a crested gecko.
Crested Geckos tails can naturally tear off as they fend off predators.
My brother picked his up on accident while cleaning his tank
& his tail was dislodged...Lol
I got a kick out of it...
My brother kinda freaked out at the time.
Good thing he has a GF
She's a little more gentle on the little guy..
Its shedding but I need to see the end --- looking for nips bites or pinches something could cause tail ro