Easing an animation running without ease

hi all,
I’m not a code expert, so I guess to find here some help.
I need some advise to build a function that slow down and stop an animation that is running cyclical (from right to left) in the back of my stage, without any other effect. The issue is that the slow-and-stop command can come at any time ('cause it will depends on user’s click on a button).
My animation is very _x-long: it something like a scene running in the bottom of the screen, and it must slow and stop when the command come,

I tried with tween class, with setInterval, but the only way I can try is valid for a short animation, cycling from right to left and then going back to its original _x position. But this is unuseful for a long scene…

please help me
thanking you in advance