Appears to be rusting of a spacer plate. I've tried unbolting the wiper blade butthe thing refuses to come so I cannot unscrew the damn plate. I'm thinknig of using rust remover in situ unless some of the more knowledge folk can help?
I believe they cleaned it and recoated all my nuts with something - sorry but I don't know what it was but it looks like yours could be a wider issue in any case ??
Best thing is to remove the plate if you can. You might be able to prise the wiper arm off the spline, or if you have a little hub-puller that would be ideal.