"Limp hair" is hair that just kind of hangs straight down. It doesn't have "body." It is usually considered unattractive.
"Floppy hair" is a description that is usually used for teenage boys in boy bands, tv shows, and movies. (when it's used to describe someone older, it means that they still have a kind of boyish charm). It's part of what makes them cute. I've never heard a woman described as having floppy hair.