As long as the nail is not hanging off and/or about to fall off, it is not considered a chatzitzah. If it is hanging off, you should gently and carefully cut off as much of the hanging nail as possible without injuring yourself.
We do recommend removing the nail polish before mikvah, as it is best to remove all barriers on the body before immersion.
Please feel free to get back to us with any further questions.
B'Hatzlacha!