Try hydrogen peroxide. Just dab at it. It just might work. Also try a magic clean eraser. Those things work miracles.

I got a pair of shorts at the thrift store that someone had shortened with some type of fusible that was coming undone and I wanted the length anyway so I was able to remove the residue with rubbing alcohol. I just saturated the area and scrubbed with my thumbnail. Any stains came out in the wash.

I would be afraid of hydrogen peroxide,i used a tin bit on a q tip to remove a stain from a table runner and it immediately pt a hole all the way through the top.

The stain may not be goo from the iron-on, but scorching from the iron
which will not come off.
I think simply sewing on the original label is best.