Can Rust Aid be used on Palm Tree trunks? My sprinkler left a nasty stain on the palm trees and I'm wondering if this is too harsh for them?

No, use something like Snowcap rust remover as it is “mostly” safe for plants, just use plenty of water on the ground before and after to mitigate damage to the roots.

Would not be good for the roots, or the trunk, I would not recomend, as you could go to any ace hardware and buy the powered rust remover that is named something like “Snow Peak Rust remover” you can also buy it online, I believe