Snoozed, now that you have been reassured, I am afraid to say that unless you regularly clean all that off, it will corrode the alloy wheels. The brake dust will lead to pitting if not removed.
There are plenty of alloy wheel cleaners from Halfords and the such like.
Just a thought˜±
I have noted over the years that the rim quality has a lot to do with ease of cleaning. Good quality alloy seems to throw the brake dust of with ease when cleaned with power hose whilst lesser quality would appear to be still showing a dirty colour no matter how much you clean. I have not found the B Max rims easy to keep clean. The design of the spokes also has a lot to do with this.
Hi ho the winter comes and the whole business starts again˜¢