A "stepper motor" is a type of electric motor that operates one "step" at a time in response to input electrical signals (pulses), and it's very common in drive-by-wire throttle body motors and other such small actuators ... including that one, which is the IAC ("idle air control") motor. The motor extends or retracts that little plunger to open up or block off the air passage from the airbox into the throttle bodies downstream of the throttle, so that the "computer" can regulate the engine's idling speed.
Somewhere down in there near that gadget, is a hose leading to the airbox. Make sure that isn't clogged or kinked or leaking or disconnected. I don't think this is the problem, because that hose is upstream of the IAC valve. If it's blocked, the symptom won't be what you describe, and if it's disconnected or leaking, the incoming air will still remain under control of the IAC even though it's not coming from the airbox.
Personally I'm not comfortable with all the black schmutz inside the opening where the IAC fits into. I'd be cleaning that up, and cleaning up the IAC plunger, and applying a thin film of grease to the O-ring before re-installing, AND making sure the IAC properly seats inside the opening and isn't sitting crooked or not seating properly.
