I would assume they are stainless steel and, yes, that is rust. It happens more with knives. If it comes off when you wipe with a cloth, that's fine. I sometimes scrub with a steel wool pad, such as SOS. But that can leave scratches.

Various sites on the internet suggest ways to help avoid the rust, such as cleaning the pieces before they go in the dishwasher, to getting them out right away. But I don't believe there is an additive that will stop it from happening.

That happens to my knives as well. It's puzzling! I scrub it off with a Brillo pad.

To remove the rust easily ,mix one part baking soda to three parts water, rub the paste with a soft cloth over rust spots, I don't like using brillo or sos pads

One thing works for me. Make sure there isn't any mayonnaise on anything in the DW.