do you have the same on your calipers?Rust is oxidised iron in the steel. It happens! If you don't like it, get it painted!
I'm using from time to time, Iron Remover for rims, but is not acidAcid wheel cleaner ?
So it is related to Iron Remover, I'm using the one from Sonax.The pattern of the rust appears to me at least to indicate that some sort of chemical has reacted with the (weak OEM) coating used on calipers. Even the iron remover I used (Gyeon) seemed to impair the OEM finish on my calipers but I did not worry since the brake dust ruined them far more!