Sounds like iron.

If you get some of the orange filter waste or goo, put it in a glass with enough water to suspend it, and add either some Vitamin C tabs (ascorbic acid) or some Iron Out. If the water clears . . . it's iron.

Likewise, if you have an orange spot you can reach, but a few Vit C tabs on it, or a Iron Out paste (in a twist of Saran wrap). Again, if it clears, it's iron.