Yes, it is rust. After your shooting session or after daily carry, wipe down your mag exterior with gun oil lube wipes (I use Hoppes 9). That should prevent the rust from spreading, but it will always be there. You might also want to check your mag's interior just to be safe.
But to get rid of the rust, you'll need to have your mags refinished. The rust has begun to eat the bare metal due to wear on your mags.