Dear Kate,
I think I would isolate her immediately as it looks like a mite problem and could be passed over if this is the case.
Poor wee thing. I have copied an article for you which I think will help.
A visit to your Vet will hopefully get you some Ivomec which is given in the form of a dropper to put onto the bird's skin. This attacks the mite through the blood stream and does work. I also use a solution of surgical spirit which I brush well into the scales to try and penetrate were the mite are buried. This is much more difficult than it sounds so I then let the leg dry for a few minutes and then coat the whole leg with Vaseline Petroleum Jelly and rub this well into the legs. The idea of this second treatment is to hopefully help to suffocate any insects you may have missed, it also helps to soften the scales and helps the regrowth process. The treatment will take quite some time to get results and will need repeating several times but eventually the scales will come off and new replacements will grow. Do not pull off the old scales as this will cause bleeding and infection.
The length of time it takes to eradicate the mite does vary from bird to bird and this is due to how badly affected the birds are. There will be a need to repeat the treatment several times to make sure that you kill the mite, the problem is that any insects which are missed will just breed and the they will reappear after just a few weeks.
Do hope this will help you. Best of luck!