If the retired federal employee income is all you have to report in AL, then you do not need to do a return at all.  If you have other income and must do a return then you do not enter the retired income.  I have attached a link to the Alabama Department of Revenue guidance on this.  See page 7.  

https://revenue.alabama.gov/wp-content/uploads/2017/08/08f40bk.pdf

If your other income is above the amount below for your filing status you need to file:

Full year residents whose filing status is “Single” must file if gross income for the year is at least $4,000. Full year residents whose filing status is “Married Filing Separate Return” must file if gross income for the year is at least $5,250.  Full year residents whose filing status is Head of Family” must file if gross income for the year is at least $7,700.  Those whose filing status is “Married Filing Joint Return” and whose gross income for the year is at least $10,500 must file an Alabama income tax return.