HoverTarget.cs

using UnityEngine;

public class HoverTarget : MonoBehaviour
{
	private Rigidbody2D rigidBody;
	private bool stationary;

	// Use this for initialization
	void Start () 
	{
		rigidBody = GetComponent<Rigidbody2D> ();
		stationary = true;
	}
	
	// Update is called once per frame
	void Update () 
	{
		rigidBody.isKinematic = stationary;
	}

	void OnCollisionEnter2D(Collision2D collision)
	{
		if(collision.gameObject.tag == "Projectile")
		{
			stationary = false;
		}
	}
}