If you have religious or spiritual beliefs or support that you can use to either bless the house, free the spirit or other ideas moms' mentioned, definitely try it. If not, I would definitely look into the ghost hunters option and see if there is someone in your area that could check things out. Either way, I would not make something up to tell her that you don't believe yourself. Especially since you seem to have your own concerns/doubts about the room. I would definitely rearrange the furniture and, if you have air in the rest of the apartment, I would remove the AC unit from her room and just leave her door open enough to get cool air from the apartment in there.
She may have just had a bad dream, but I don't know where she would have gotten the idea to even process into a dream. Definitely look into the shadows of the light or night light, a sweet big-eyed stuffed animal in the light, becomes a terrifying being in the dark.
Also, kids your daughter's age start pulling every delay-tactic invented when it comes to going to bed. And while this episode at 3:45 definitely wasn't that, the outcome of sleeping on the couch may become a request every night from now thru college, so you want your daughter to feel safe sleeping in her room, and to get back into it as quickly as possible.
I don't know if ghosts exist, I'm thankful I haven't seen one, but I've never written them off completely. Good luck! Just do what you have to do to comfort your daughter and keep her safe.