Not sure, but it looks like fluid retention, I don't think there's a lot you can do other than wait and see what happens, It may rectify itself, it might not. Newts lay hundreds of eggs and unfortunately not all of them can thrive. I've seen it myself where apparently healthy larvae and new morphs develop strange problems or stop eating etc, although it can be upsetting I just put it down to nature weeding out the genetically weak individuals for the good of the species.