There are different grades of stainless steel. Lower grades contain more carbon than higher grades, and can definitely rust
Rust will wipe off in many cases, at least some of it. Diatoms should come off completely leaving a clean surface. Rust would leave a stain to some degree.
Having said that Im not sure a little rust would have much of a negative effect on livestock, a lot of rust maybe. So your problem there may lie elsewhere.
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