updated RotateTowardsMouse. You might want to try out with and without using slerp (useSlerp boolean) and by using directly the Input.mousePosition as the lookDirection

Assets/Scripts/RotateTowardsMouse.cs

@@ -2,14 +2,45 @@
 using System.Collections.Generic;
 using UnityEngine;
 
-public class RotateTowardsMouse : MonoBehaviour {
-    //Circa qualcosa così
+public class RotateTowardsMouse : MonoBehaviour 
+{
+	/// <summary>
+	/// Velocità a cui far ruotare il braccio.
+	/// </summary>
+	public int speed = 1;
 
-    void Update() {
+	/// <summary>
+	/// Per testing.
+	/// Determina se bisogna utilizzare 
+	/// Quaternion.Slerp() o Quaternion.LookDirection().
+	/// </summary>
+	public bool useSlerp = false;
+
+    void Update() 
+	{
         //Trova la direzione tra il transform e il braccio
-        Vector2 screenPosition = Camera.main.WorldToScreenPoint(transform.position);
-        Vector2 direction = (Vector2)Input.mousePosition - screenPosition;
-        //Cambia la rotazione dell'oggetto
-        transform.rotation = Quaternion.identity;
+        Vector2 screenPosition = 
+			Camera.main.WorldToScreenPoint(transform.position);
+		
+		Vector2 direction = 
+			(Input.mousePosition - screenPosition) as Vector2;
+
+		Vector3 lookDirection =
+			new Vector3(direction.x, direction.y);
+
+		/*
+		 * In alternativa provare direttamente con 
+		 * lookDirection = Input.mousePosition;
+		 */
+		if (useSlerp) 
+		{
+			Quaternion.Slerp (
+				transform.rotation, 						// Rotazione iniziale
+				Quaternion.LookRotation (lookDirection), 	// Rotazione finale
+				speed * Time.deltaTime);					// Velocità dell'interpolazione
+		}
+		
+		else // Questo dovrebbe bastare
+			Quaternion.LookRotation (lookDirection);
     }
 }