It is factory sound deadener/chip guard/weatherseal. It's unsightly but reasonably effective for road stone noise dampening.
If you leave it on you can hit it with underhood black after washing with a soft scrub brush and letting it dry. There are a couple of brands of coatings designed to freshen it up but if it's otherwise in good shape just make it black again.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
