My mother experienced something very similar to this several years ago. Mom had been in bed reading, and said that she was fully awake at the time. She was not the type to be overly imaginative, so when she said something happened, everyone believed her.
We were living in a beautiful Victorian home in a small town and had occasionally experienced some paranormal activity.
Oddly enough, on Halloween of one year, Mom saw the head of a woman, about 35 years of age, with a Civil War era hairstyle. I know it sounds really trite, happening on Halloween; I guess it was just weird timing!
We found out later, that the house we had, which was built at the turn of the last century, had replaced a plantation home on that site.
Friends of mine who lived a block from us, had the apparition of an older, African American, slave woman appear to several members of their family in their kitchen. Their home sat on where the slave quarters had been.
My guess would be that you encountered a spirit. Ghosts and spirits take various forms, shadows, mists, full apparitions... I've wondered if it took spirits perhaps more energy to manifest in full body, and those who did not have as much energy where only able to manifest parts of their body. I've got a friend who swears that on several occasions he witnessed a man in jeans running from his backyard to the lake that backed up to his property -- only the jeans were visible.
I know this had to be a bit of a fright and at minimum really perplexing, but it would be interesting to see if this happens again. It makes me wonder who he was talking to. Please keep us updated!