Probably paint failure around the screw and/or loose or overtighten screw causing paint failure. Very typical of aging cars.
Take to a reputable body shop if you don't want to fix it yourself, shouldn't be a big deal. You can remove the trim, lightly sand, coat with conversion primer (or etch primer, same thing) -- this chemically coverts the rust to inert iron phosphates or similar and will prevent future rust. Then pain with factory type primer and paint. have a care that the body shop doesn't paint half the car to "match" the paint! You only need a small repair and "perfect" paint match isn't necessary on an "non-show" car.
Other spots to watch are the corners of the fenders, where road dirt and mud collect, and the base of the rear window gasket. Most of the rear window gaskets on non-garaged cars are in bad shape by now -- you can check by lifting the edge with a fingernail. If it isn't soft and doesn't snap right back down, or shows signs of cracking or rubbing the paint, get it replaced.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!