I don’t know exactly how this works in your state, but in general state Medicaid agencies may only seek recovery of the expenses they paid from the property of the deceased beneficiary. In your case, you say that your husband left no assets to you. Therefore, there are no assets against which the state can assert its claim. They cannot touch your Social Security. To confirm this, I’d recommend calling the office that sent you the letter.
For more on Medicaid's power to recoup benefits paid, click here.