I usually just rake it to spread it out and turn over the stones, or you could just remove the offending gravel. I've never thought to do it, but I suppose you could try to wash it off with a powerful de-greasing cleaner. I have a concentrated one for my boat that I got at a marine supply store but they probably have something similar at your hardware store - just ask the clerk. I know that one of the best de-greasing, yet mild soaps is Dawn dishwashing soap. They use this for crude covered birds in oil slicks and I also have the inside skinny from a grass/golf course expert that one shot glass of this in a one gallon sprayer of water is the ultimate moss killer for lawns (more isn't better just a couple ounces in a sprayer left to dry on the grass).