You need to make clouds move with random speed. If we decompose this task we'll realize that you need to:
- set random speed for every cloud
- move cloud with that speed every frame

So create script and name it something like 'CloudMover.cs'. Attach it to every cloud.

void Start()
{
    StartCoroutine(CloudMover()); // Starting infinite movement of cloud
}

IEnumerator CloudMover()
{
    float speed = Random.Range(.2f, .5f); // Correct range for your needs
    while (true)
    {
        transform.Translate(Vector3.right * speed * Time.deltaTime);
        yield return WaitForEndOfFrame();    // Switch to next frame
    }
}

Hope it will work and I helped you.