There's something wrong with my socket?

I have an older house, so we used one of those in window air conditioners. We had ti plugged into the socket, but unplugged it just recently cause me noticed something. Under the socket there's this copper-colored stuff that oozed out. The liquid is watery and looks like a dark colored paint that dripped down from the socket to about two inches below. Is this a problem? Can we still use the socket? Whats wrong with it?