As Moira, can you cancel your own ultimate ability?
No you can't cancel your own ultimate, apart from killing yourself.
Enemies such as McCree and Hog can cancel the ult when using some of their abilities on you, but Moira can't do it alone.
No, there is no way to permaturely cancel Coalescence as Moira. It will only stop at the end of the ultimate's duration.
That said, Coalescence has a range of 30 meters and grants Moira a noticeable speed boost, which makes it easier to stay out of harm's way and retreat behind a corner in case of a Self-Destruct or Deadeye.