The lid from a coffee cup spent a cycle melding with the circular heating element at the bottom of my dishwasher. While it wasn't too hard to pull it off, there is still some plastic and rubber gasket adhered to the metal. Googling this suggests scraping in some form or another with various kitchen tools, but as the element feels just like a non-stick frying pan I'm hesitant to go that route without further input.
I've used a butter knife. It seems like the melting process hardens the plastic and it flakes./breaks off quite easily.
Think of the heating element like a burner on a simple electric range. They are exposed, and metal pots and pans rub on them all the time. The element doesn't have a "coating" that you're going to scrape off like a non-stick pan.
Try to get off as much as you can. Every little bit that you leave on is going to melt and smell bad again each time you wash until it's gone.