Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Distraction; Confusion
ATC Issue All Types; Conflict NMAC
Air Traffic Control Issued Advisory / Alert; Flight Crew Took Evasive Action
Environment - Non Weather Related; Human Factors
We were descending on the ZZZZZ RNAV arrival landing [Runway] XXR in ZZZ; either just prior to ZZZZZ1; or just after; level at 6;000 feet.  We heard the Controller talking to another aircraft; Aircraft Y also at 6;000 feet.  That aircraft was issued a descent; but did not comply.  A few moments later the Controller asked if they had us in sight; and told Aircraft Y to descend immediately; and if they did not see us; to also turn immediately.  This is about the time we received the TA; and saw them ahead of us and at our altitude; on TCAS; then we received the RA; I executed the TCAS escape maneuver; TCAS issued a very steep climb; close to 6;000 FT/min.  The aircraft passed beneath us at 150-200 feet. I would call this one a near miss.I'm not certain whether the Controller lost situational awareness with this GA aircraft; or whether the pilot of that aircraft was not properly following instruction; or possibly both.  We were at our last assigned altitude; on the arrival in level flight.The Controller appeared to be working two frequencies; and seemed task saturated.  Again; I'm not sure whether the Controller lost sight of the situation; or whether this extreme RA was due to the GA pilot not following Controller instructions.
First Officer reported a near miss during approach with GA traffic.