The Steering stem assembly is very similar to those found on bicycles, although the Slider has an additional 30+ inches of steering tube that leverages against the steerring stem bearing assembly that may work loose slightly over time causing this end play (wiggle) you are experiencing.

a bike shop would use an adjustable Crescent wrench or locking pliers to loosen(counter-clockwise) the top Hex Jam (locking) nut by about 1/2 turn to make room and ease the adjustment. Locking pliers or strap wrench will then be used to tighten (clockwise) the knurled adjusting collar approx. 1/16 of a turn to eliminate the tube end play (wiggle) - if too tight ten then it will beging to bind the steering. Tis then tightened into and against the knurled collar to lock the adjustment.