ufo/Assets/Script/BGTransform.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class BGTransform : MonoBehaviour
{

    public float slipSpeed;

    public float tileSizeX;

    private Vector3 startPosition;

    // Use this for initialization
    void Start()
    {
        startPosition = transform.position;
    }

    // Update is called once per frame
    void Update()
    {
        float newPos = Mathf.Repeat(Time.time * slipSpeed, tileSizeX);
        transform.position = startPosition + Vector3.right * newPos;
    }
}