I think using folklore and cultural mythologies is a great way to add depth to a story in an interesting way! Rather than off putting to non-Koreans, I think many people would be very engaged in this, as long as you can deliver in a way that's not confusing. Not everything needs to be 100% explained, mind you, so don't feel like you have to completely hold your readers hands through it all!
Some of the points about Korean folklore happening in America are good points, but I have to say, I don't think this would bother me at all? Especially when it sounds like the main characters are Korean. While magic being location specific is an interesting thing to explore, there's no reason it has to be location specific. Maybe this magic is everywhere, and your MCs are sensitive or notice it because it is more culturally relevant to them! Whose to say? That said, I think @joannekwan 's suggestion is already pretty perfect (and I do see you responded to her, so you're obviously already thinking about this! lol )
But yeah, speaking on the main thing. If it is something you are interested in exploring in your story, I wouldn't personally be put off by that kind of thing. I think it's great. =)