I’ve read people use tomato catchup but that was mainly for the natural yellowish stains. Maybe it would work for this type of thing as well.

 

Have you tried soaking in white vinegar? Leave it in for 5 minutes, then rinse out and let the sun do the rest. Not promising it will work, but it really whitens our greys' tails.

 

Try the Orvus on him, it should work..

 

My pepper gets the same thing Urgh and even after a bath. I found the purple shampoo does the trick. At least so far it has worked

 

Dry cornstarch (from the kitchen cupboard) is what we always used for yellow stains on the legs; I don’t know how it might work on metal stains.

 

That is not what I was imagining of "rusty" but more a grease stain.
Metal is coated with not sure what to inhibit rusting, so that may be removed from say the top from hands or things touching, but the underside is still coated and by looks of it, your horse is wearing it.
Sometimes our bodies "PH balance" will do that as a interaction with certain metals too.

If what others suggested not work, a degreaser product found in any auto section or specialty store may work, but it is caustic and tough on the hands and in this case mane.
Make sure to protect the horses neck as much as possible and use very lightly and only where the issue is. Wash with soap after to remove it and if this were my horse his entire neck would have a bath, actually if going to do that just give him a bath but concentrate on that mane 2x....

Before doing anything "caustic" though, blue Dawn dishwashing soap is a degreaser but more hand and skin friendly it might just do the trick. I would try that....
Orvus would also be my next go to....it just cleans the clean cleaner I've found.
Let us see how it looks once you're done.
🐴.. jmo...

 

Have you tried the green spot remover? I have gotten to wear I hall with a sheet because mine gets a stripe along his side from the aluminum brace on my divider. I can spray the green spot remover on to and it does the trick but I haven't had to do it on a mane before.

 

I think is worth a try that green spot out does good