Ways to counter Moji
1) Shields to block her spray from creating marks
2) Vertically mobile champs that can take high ground and stay out of her range
3) Champs with long-range poke and/or decent mobility in general
Use your grenade to hit behind her shield; if she turns around to block it you can get free shots in. Don't fire on the shield unless you have high Wrecker, because she can hit you during that time. Once the shield is down, fire away.
Use the totem legendary to give your green circle cc immunity. Deploy to heal and hop inside when the grumpy bomb is near you. Grumpy bomb does little damage so the totem essentially negates it.