Might not be rust. Could be burns. Either way wouldn't be the first Star Trek ship with that detail, Klingons ships have damage like that.
Looks like the Eleos is Phoenix class, named after the John Eaves concept art it was based off https://twitter.com/DaveBlass/status/1629978293112897536?s=20
It would depend on why she left starfleet, and what rank she was when she left. Looking at screen caps of the actual Eleos and not the concept model, it doesn't appear to have any of that 'rust' in the final show model.
It'll be interesting to learn in a couple of years what Blass, Drexler, and Krause have to say about the whole design process when they can speak more freely.
Dave Blass posted more information on the Eleos on his Twitter account. Apparently the Eleos is a Phoenix-class ship, "designed as a small rapid response vessel ... the Phoenix-class was a move towards higher warp speeds in smaller designs." The warp field governor is key to such relatively high speeds in a small ship.