The crashed car in the picture looks like it was melted by slime.
Not that I've ever heard. This is just a weird headline.
Not generally, no, although I am reminded of a style of melted-cheese-coated sandwiches called "melts".
In that context it's the cheese that's being melted, not the sandwich. Melting implies some kind of state change from solid to liquid (or semi-liquid), which is in fact the case in a melt sandwich where the cheese is melting. In this case the slime was already in a liquid state so there's no change taking place and it would therefore be incorrect to refer to it as melting. It DOES look kind of like the car was melted because of the accident damage combined with being covered in slime, which is I guess what the title is alluding to.
Nothing is so small that Hacker News cannot overthink it, not even a cheese sandwich.