Does this character sound scientifically possible? I know it's not too realistic, it's sci-fi. LONG question?

It sounds rather like the T-1000 from Terminator 2. It could be a cool premise on which to base a story, but it's not remotely scientifically plausible, because mercury and most of its compounds are toxic to organic life. If the creature is made (almost) entirely out of mercury, that doesn't allow for sufficiently varied chemistry for making and controlling a body. If it can't eat, where does its energy come from?