Sounds to me like he's trying to shed his skin. They have a very thin, slimy outer layer of skin that protects them, and shed it fairly often. When they do, they rub their body all over, and can contort themselves up a surprising amount. Sometimes they can look almost as if they're choking or straining hard. When he's done, there will be a string of slimy skin stuck somewhere - which can be incredibly annoying to clean off if they happen to stick it to the tank glass.
Making sure he has adequate humidity by misting frequently, and access to a bowl of clean, pure water he can sit in will help him along with the process... but these are things a White's needs all the time anyway.
She sits in her corner, singing herself to sleep.
Wrapped in all of the promises, that no one seems to keep.