Typicly plants that have red or purple leaves will absorbe iron througt the soil. If you ever have aquarium plants or even some house plants that are red you will want to add some iron into the water.
With a canabis plant I dont see a problem with a little rust in the pot. All plants need a small amount of all nutrients, that includes iron.
The worst thing that I could see happen is that you might get some reddening of the leaves and stems. But even if that happens you wont notice any thing in the end result.