Triturus alpestris belly turns white

[ester]

I was cleaning a few aquaria this morning when I noticed that one of my triturus alpestris adults has a near white belly. The picture isn't as good as I'd like but what's visible is a white belly with some pale remains of yellow.

Any clue what caused this or what I should do?
The others seem fine, they've all been fed on red mosquito larvae and earth worms.