Most livebearers are born with an yolk sac and this is what feeds the fry for the first 24 hours or so, sometimes it can take a while to go though. It can also sometimes cause difficulty swimming if the fry was born premature or is the runt, in which case his/her yolk sac will be bigger and will definitely take longer to go.
I've had a few livebearers born like this and they all grew up to be healthy adults, it just happens sometimes. If it's getting better then it will definitely go away, it may just take time.