Hi Susan --
This is not a nice way to wake up! The good news is that it was all just a dream. There really is no monster that wants to eat your mother!
Because water in dreams symbolizes emotions, we know the frozen lake in your living room represents feelings about your family (the dream is set in your house) that are "frozen" -- that you are not allowing yourself to express. Given the background information that you provide (you describe your mother as controlling), is it possible the monster that lies frozen at the bottom of the lake represents your anger at your mother that you are afraid of expressing?
Violence in dreams is a symbol of frustration. Accordingly, the monster that is unleashed should not be interpreted as a literal desire to hurt or injure your mother. On the other hand, your dream does show that sometimes your anger reaches a boiling point. Have you thought about ways that you might be able to speak to your mother -- to let her know that her "little girl" is already grown up and ready to make her own decisions?
In the meantime, you may need to sit down with a good friend and begin working on melting some of your "frozen emotions." It will feel good to let your feelings out, and then you can begin working on new ways to communicate better with your mom!