Rust in salt, is it safe

Are you trying to make Sauerkraut?

Either way for the pink colour you have a non-desirable bacteria growing. While most Sauerkraut has some of this, it is not always safe to eat. Time to throw it away?

If you salt cabbage, you need to ensure not too much salt is used (1% to 2% max), and make sure it is packed down very firmly, so only anaerobic bacteria will grow in any numbers

It doesn't need to be in a closed container, but it should have a firm fitting lid, without an air gap on the top

Also, once it goes past 20°C (70°F) you run the risk of getting other undesirable bacteria and yeasts, and general food rot

If it's only rust, you should be fine. Rust is oxidized iron and harmless. Check for other discolorations or changes in surface texture, that means something else is happening there If in doubt, ditch it.

Here's what I'd do: I'd cut off (rather generously) the rusted parts, sample a bit of pudding from the inside and decide if the recipe is worth another try in a new mold. Then, get yourself a new mold. This time, make sure it's appropriate for your intended use: Some puddings are cooked in the mold, then served right away (so no danger of excessive rusting), some , like your plum pudding, need longer time to "ripen". Maybe you coud get a porcelain mold?