In Michigan Friend of the Court handles child support. As the other poster stated, FOC does operate under the assumption that the receiving parent of support still provides for the child even if the child is living with another caretaker.
So your going to have to Motion to Modify the current support order through Friend of the Court.
As the other poster also stated it's summer and perhaps the child is just visiting Grandma, so you will want to make sure of that, before writting Friend of the Court.
Even if the child is living with Grandma, you will still most likely end up paying mom or grandma child support. Michigan does have 3rd parties receiving child support. Someone is supporting the child and they expect you as the parent to pay them to. You can read up on 3rd parties and child support in the Friend of the Court handbook. You will want to look in the FOC handbook for the county that your order is out of.
Seems to me your only concern here is money, and not the fact that the custodial parent has sent the child off to live with a 3rd party. FOC will notice this fact to.