It looks like it could be a prolapsed vent. That white stuff covering it is just a substance that seems to ooze out a lot during a prolapse. Here's what you should do:
Give her a bath. Fill a bin, bucket, etc with HOT water (95 degrees). Submerse her lower half, and let her soak for 20-25 minutes. If possible, use your hands or a soft cloth to gently try to rub the white stuff away. This will take some doing, as it can dry like concrete, but be gentle--too much irritation will just make the prolapse worse. Give a bath to her every day or so to prevent accumulation of that white stuff (I'm not exactly sure what the "white stuff" is, but it occurs when birds have prolapsed vents).
After bathing, gently try to push the prolapse in. It is unlikely to stay in very long, especially if you haven't removed all of the white discharge yet. Then, apply some honey and some Witch Hazel. Some people also use PreparationH. All three of these items will help shrink the prolapse and reduce swelling. Continue doing these things (pushing it in, putting honey and Witch Hazel on) every day.
Hopefully, she will recover. I once had a bird with a prolapsed vent, and through doing the above procedure, she recovered successfully. Some people say that a bird with a prolapsed vent will have that same problem again in the future, but my hen has been perfectly fine ever since and has laid some really large eggs.
