Moon Knight Ultimate

Been trying to learn to heal with invisible woman. Is Moon Knights Ultimate reactable at all? I've been trying to use my Ultimate more strategically, specifically at key pushes or to counter enemy ults. Moon Knight seems like me or my team are literally dead or as good as dead before the audio cue plays. Is there a way to tell this is coming, it seems very important to know especially since he seems to build ultimate charge twice as quickly as anyone else