What's wrong with this?

I just noticed that an electrical outlet in my house looks very rusted. I don't think it's rust because it's sticky and rubs off. It has melted the area it was touching on the plug. It is the outlet that my window a/c unit is plugged into.

I immediately turned off the unit and unplugged it and the plug is like that too. There is no way that anything could've been spilled onto it because of the location. What could be wrong?

I want to find out quickly because I'm not turning it back on until I do and it's getting hot in my little house!