To be honest, that doesn't look like rust.
Please check your brake fluid ASAP!
It appears only on the trailing edge of the spoke - means it's spraying/being ejected from the inside of the wheel. The distance from the hub is about equivalent to the inner edge of the caliper. It could be a bad brake line or seal....
Is the material thick (highly viscous)? or thin? If thick, it could also be a bad wheel bearing seal. If thin, it's more likely brake fluid.