The substance that is leftover were rubber arms that move the coins around as the hopper plate spins. It is designed so that the coins don't stick to the hopper to the side where the hopper wheel cannot pick it up. Without a complete agitator, you may get hopper empty errors before it is actually empty, but if you keep a couple hundred coins in your hopper at all times, it shouldn't be an issue. So if you cover it up, but keep enough coins all the time, it should not be an issue. Your pictures are kind of blurry, but you should be ok removing the gunk or covering it up. There's is a universal slot specialist here (uniman), maybe he can chime in as my hoppers have a complete rubber star to agitate the coins, you have some sort of metal part with arms swinging out. I've been trying to find a picture of a "healthy" hopper of your type, but can't find one.