I'm so sorry your little one isn't doing well.
What you are showing in the picture is actually her umbilicus (belly button) you don't want to pull that off. Just like the bit of an umbilical cord that is left on babies it needs to dry and fall off on its own.
Her vent should be right below her tail, do you see it when looking for it?
Coccidia is a big killer of chicks and often the first sign is lethargy and generally doing poorly. If you see any bright blood in the stools that means you are most likely dealing with coccidiosis, blood won't always be present though. The easiest product to get to treat it is probably Corid (amprolium). Usually at that age if they are doing really poorly they don't pull through, but it's worth a shot and the rest will hopefully be prevented from going downhill with it as well if you treat them all. If you decide to give it a try make sure not to give any extra vitamins, Corid blocks thiamin to starve the coccidia and if you give excess vitamins it will prevent it from working. Once treatment is complete you can give them some extra vitamins to replenish their thiamin.
Sometimes there are just things that are wrong with chicks and they don't make it past their first week though. It could be that this little one has something wrong internally that can't be fixed.
Good luck, I hope your little one does pull through! Hopefully you'll get some other opinions as well to decide on the best course of action.