Just my personal opinion, but based on the description of her skill it should be "Intangibility" instead of "Unstoppable". She's invisible while still able to touch objects, and she's undetectable while doing it as well. That's why monsters lose aggro on her and she can just walk past them.
To me "Unstoppable" will always be associated with Juggernaut-like powers, where you're visible to the naked eye and detectable by the rest of the senses but nothing can physically impede or harm you.