[dirty hands]

that sounds like clear coat overspray
is your car paint metalic or acrylic lacquer

if it has been resprayed in two pack I dont know what chemicals are in that

then ask for a clear coat /acrylic lacquer thinner to remove this layer
it will not come off easily at first because it has aged and hardened
the Thinner reactive on some rubbers not all
before you use it on the outside of the mudflap
first test a small amount on a rag
in the inside of the mudflap in case it bubbles /reacts