Substrate

If the metal is oxidizing, then u might want to remove that metal part out of there, and any contaminated cocofiber, as ur t might drink from it if water hits it at any point.

If you really must have something rusty in there, perhaps you could try a few coats of food-grade spray lacquer so that it doesn't continue to oxidise/leech into the substrate. I understand the desire for aesthetics, but certainly not at the expense of your T's health. I would imagine any food-grade lacquer/varnish would be fine, but don't quote me on that.