It is normal for the IAC to slowly close down the RPM on decel. This is the dashpot function and it helps prevent hydrocarbon spikes when the throttle plate is suddenly closed and the vacuum jumps up.
If it is hanging longer than it should like yours seems to be, there are three common causes:
IAC valve is sticking open
TPS voltage is too high with the plate closed (0.50v~1.25v is normal)
Throttle plate is sticking open
**Edit** Here are the meanings of the acronyms above:
IAC = Idle Air Control valve
TPS = Throttle Position Sensor