Indigo Dotty back w/ a bubble on its jaw

[acrofreak]

Hi i got a indigo dottydack and it has some sort of bubble eon its jaw ive seen this on a perc in my lfs but i dont know what it is and what to do about it. He has been hiding so i cant get a picture for you guys. He is very camera shy. Any help would be much appreciated. Thak you