Rust on unknown metal cooking mold
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?