GAC freaking me out

[Brisc0]

Morning/Evening everyone, I am a little disturbed. My GAC filter bag is just a standard drawstring media bag for aquarium use, however I am noticing today that it has the appearance of rust on it. There are two large spots on the outside of the bag. Anyone seen this can tell me what is going on? Surely it can't be rust...

 

[ReefRaff]

Maybe a brown algea, or red cyno growing on it?

 

[Brisc0]

no, its like the white fiber of the bag has turned a rusty orange. The stain is permanent.

 

[WarOrks15]

but what would rust in the spot it was sitting at.