Hi ant. Welcome from the Pacific NW.
Skin irritations/pimples/infections on chins are quite common. And most often the problem (or at least the contributing problem) ends up being the feeding and/or water dish. A plastic slow feeder is just about the worst possible culprit that it can think of. Perfect breeding ground for bacteria. Even if you think that you are cleaning them well.
Switch to stainless or ceramic bowls. Wash the feeding bowl thoroughly between meals, and the waterfowl at least once a day. Put both through the dishwasher, if you have one, every once in a while.
I would gently clean the area with a very mild soap after eating do not leave any soap residue. Some people recommend using human acne tips or pads. They contain Salicylic Acid. I prefer Chlorhexidine wipes which are readily available over the counter at pet supply stores. Just make sure that if you do use them that you make sure the dog doesn't lick its chin until the area is dry.
Hope this helps