lerp(player.x, player.x-200, 0.5) is the equivilant of moving the player left 100 pixels every time it is run.
Since player.x gets updated every time it is moved, lerp isn't doing much here.
Example if you have running every tick
Frame 1. Player.x is 2000. lerp(player.x, player.x-200, 0.5) = lerp(2000, 1800, 0.5) = 1900
Frame 2. Player.x is 1900. lerp(player.x, player.x-200, 0.5) = lerp(1900, 1700, 0.5) = 1800
Frame 3. Player.x is 1800. lerp(player.x, player.x-200, 0.5) = lerp(1800, 1600, 0.5) = 1700
and so on.
If you want a gradual movement that slows down, use a fixed target (usually accomplished by setting the target position in a variable once). So set TargetPosition to player.x-200, one time. Following the previous example on frame 1, this would be 1800.
Frame 1. Player.x is 2000. lerp(player.x, TargetPosition, 0.5) = lerp(2000, 1800, 0.5) = 1900
Frame 2. Player.x is 1900. lerp(player.x, TargetPosition, 0.5) = lerp(1900, 1800, 0.5) = 1850
Frame 3. Player.x is 1850. lerp(player.x, TargetPosition, 0.5) = lerp(1850, 1800, 0.5) = 1825
and so on. You can see the amount the player moves each tick gets less and less, but it will never actually reach 1800.
If your lerp event runs only once, it also doesn't serve much purpose in this case.