If what you are talking about is from the butt area then they did not absorbe all the goodies from the egg while developing. If the goslings seem OK then it should be OK. If the remaining yolk sac is hanging off gently wipe the remainder away after a couple of days. Don't worry too much unless you notice them being lethargic and not very mobile. If they are walking around and drinking and eating normally you should be OK. Sometimes it can be nerve racking to see the remaining yolk sac but if they are mobile and eating you can be a proud gosling owner. Hope this eases the anxiety a bit. Steve