Is that a stain or an iron burn? If it's a stain, need to know what kind of stain it is for the recommended best stain removal approach. Or, just use Spray-N-Wash or similar and see what happens. IDK how to get rid of iron burns, or if it's even possible. Wish I did, because I have that on a favorite pair of pants of mine.

invisible residue detergent or some washing aid that remained on the pants that appeared once the heat from the iron activated it.

^^ Do you know how to get rid of it, Beverly? Or is that sort of thing permanent once the iron hits it. My marks never came out upon re-wash, and the pants have been washed countless times since the marks appeared.

Sorry it was my best guess, but only a guess.

When I have a stubborn stains on clothes I rub bar soap on the problem and throw it in the wash. It's usually more effective than any of the sprays.

If you put the pants into the dryer with the stain not totally being removed, you now have set the stain.