SlidableMask.cs

using UnityEngine;
using UnityEngine.UI;

public class SlidableMask : MonoBehaviour
{
    [SerializeField]
    private Slider slider;

    private RectTransform rectTransform;
    private Vector3 farLeft;
    private Vector3 farRight;

    private void Awake()
    {
        rectTransform = GetComponent<RectTransform>();
        slider.onValueChanged.AddListener(HandleSliderChanged);

        farLeft = rectTransform.position - new Vector3(rectTransform.rect.width, 0f);
        farRight = rectTransform.position;
    }

    private void Start()
    {
        HandleSliderChanged(slider.value); // This needs to be called AFTER RectTransformLockPositions's Awake().
    }

    private void HandleSliderChanged(float value)
    {
        rectTransform.position = Vector2.Lerp(farLeft, farRight, value);
    }
}