The Moment had a conscience & was trying to save the Doctor from the consequences of his own despair, his loss of hope.
In "The Parting of the Ways", the Doctor had lost hope of protecting Earth & its inhabitants from the Daleks. Rose Tyler saved him from the consequences of that despair, so the Moment chose to appear as Rose did at that specific time -- that is, to appear as Rose when she was the Bad Wolf entity. As the Moment said, "I chose this face and form especially for you. It's from your past. Or possibly your future. I always get those two mixed up."