Hi there, just keep an eye on it, it's not something that can be definitely identified yet, it may just be a kink from a bite or other trauma. You can dab betadine on it every day as well as neosporin. If the area gets worse + climbs up the tail then you'll probably need a vet. BTW, it's best to keep the babies in separate tanks if they aren't already. How are the little guys doing otherwise and do you have the right type of sset up ? Can you post a few pics of their tank + lights ?