what are Registered Nurses that work in the ICU called?

There is no specific names; they are all RNs and then they could say ICU RN or RN in the ICU--there just is no standard specific designation, some call them critical care RN, that is probably the most common.

They are called ICU nurses, but they often have certification as critical care nurses.

You would be
Chris Smith, RN

Possibly
Chris Smith, RN-BSN (or some form of that)

On a resume, the RN would already be at the top with your name, so the icu part would be under job experience.

Anytown Regional Hospital
2000-2016
ICU
Rotating Charge Nurse 2012-2014
Efficiently ran a 30 bed unit...