Don't mean to make you paranoid but tail rust can get worse, personally I take the head off and replace the rusty parts and if there is any on the tail I use pure acetone and scrub it off with a cotton bud or too works really well, it's not as hard as it seems, just see how the rust go's if it hasn't got worse in 6 months, I doubt it ever will.
But if you don't want it there, try removing rust on a few baits first.
Do you live in a really hot part of Oz?
Tail rust worsens here because it gets so muggy in summer and the moisture in the air helps accelerate the rusting.
I usually get the head off and scrub them clean with an old toothbrush inside.
I don't think it will get worse but it might be harder to get it of. Even a pony with a little rust shown on the outside can look really disgusting on the inside.
In worst case the rust can discolor the plastic from the inside(I've seen that). I say it always best to remove the head of a pony with tailrust and fix her up. Throw away the washer and scrub the rusted area with toothpaste.
If a pony has tail rust, there is a very good chance that it is moldy inside. I would most definately pop off the head to check. Mold will discolor the plastic, as can a rusty washer. I use soft scrub with bleach to clean my ponies and remove tail rust, but whitening toothpaste should work too.
The rust does get worse, after it's eaten the washer, it'll eat at the clip holding the hair. I had a baby CC with bad tail rust, her tail was almost brown but I got it out, however, her hair clip was almost totally eaten away. I'd go in and take out the washer.