I'm fairly sure this is a bug and not intended because it's not in Twisted Advance's tooltip. But if Moakia at the perfect time he can negate damage. So far I've only seen it against Nunu's ultimate Absolute Zero.
Steps to reproduce:
During Nunu's ultimate Twisted Advance to any nearby enemy
Moakia will take no damage
I don't think the word "untargetable" is used... something about a "transforms into a cloud of energy". I'll speak to Volty about what he thinks the tooltip should contain. Thanks for pointing out that this wording might be insufficient to convey the what the ability does.
Bug - Moakia's Twisted advance negates damage
Steps to reproduce:
During Nunu's ultimate Twisted Advance to any nearby enemy
Moakia will take no damage
You can also use this to "reset" turret aggro off of you and onto another allied unit.