If I understand correctly, in one or more past years you somehow paid more real estate tax than you actually owed. The settlement that you have now received is a refund of some or all of the excess real estate tax that you paid. Is that correct?
Did you claim an itemized deduction for the overpaid real estate tax on your federal tax return for the year (or years) that you originally paid it? If not, you do not have to report the settlement as income. If you did claim a deduction for part or all of the overpaid tax, it gets more complicated.