Fixing Rust Spot on the Finished Wall

Carefully cut around the edges of the rust spot.
Try to remove the residue, if possible.
Use a thin construction tape in reasonable size to adhere over the rust spot.
The tape should nicely cover the entire spot, to prevent the rust from resurfacing.
Now, use the leftover mixture from the days before. Very little is required to cover the spot. If the mixture is too dry, sprinkle some water and mix again to become paste, then carefully patch onto the spot.
Smooth over the edges carefully with the tip of the trowel until the surface is flat and blends with the entire wall surface.
Walla! Can't tell it's been repaired.
PS: Always always remember to keep some leftover material in a nicely sealed bucket or bag eventhough application is completely done. Some defect cannot be seen until the wall is completely dry in the days to come. This is to assume that the wall underlying surface may be problematic.