Is your daughter sleep deprived?
Does she take any medication?
Does she read a lot into the subject / talk a lot about the Hat Man ( with friends maybe? )
Is she suffering from any mental condition?
I hope none of my questions offend you, but if she reports the sightings to be so real, it must be due one of the reasons above.
Here in my country ( Romania ) , the 'shadow people' are represented by only one major figure: Baubau. Parents use this name to scare off their children if they attempt any dangerous deeds( eg: Don't go so close to the river bank , Baubau is lurking over there , or Don't go in that abandoned house , Baubau is there ) and many kids develop a fear of this 'Baubau' which they usually get rid off only in their late childhood. This Baubau's behaviour differs because of the way their parents imply it ( some are evil , 'killing' you if you enter their 'den' , some will kidnap you , some will laugh at you , some will slap you and so on ) and every child who's been scared by their parents with this 'Baubau' thing swears to have witnessed such particular events.
IN CONCLUSION : Don't worry , there's no such thing as the Hat Man. You should probably talk to her , telling her that the Hat Man is just a children story, and that the more she thinks of it , the more she'll see it. If it affects her social life at all , take her to a psychologist, but I think that you'll do fine by yourself. After all , she's still a child.