water splotch on screen

today, I was using my late 2013 13 inch macbook pro when the screen got dirty. I decided to clean it with a damp towel but when i got done, to my dismay, it was like this:there was a considerable splotch on the screen, and it wouldn’t go away. I let it dry in my closet, but i decided to check on it, it had gotten considerably worse.

Does anyone know how to help? I’ve only had this for a month, and I don’t want to pay a large amount of money for this. I know that AppleCare isn’t an option, as this computer is nearly 6 years old, so I’m reaching out here for help. if anyone knows a solution that would be fantastic. Thanks!