If his parents were married, it would have said "Mary Munro M.S. Alexander".
1881 at Lower Ashalloch croft, Forgue:
Alexander Alexander, 53, crofter of 12 acres, b. Inverkeithny
Eliza Alexander, 42, crofter's wife, b. Huntly
Alexander Munro, Daughter's son, 1, b. Ythan Wells
1891 at Howe of Ashalloch, Forgue:
Alexander Alexander, 60, crofter
Elizabeth Alexander, 51, crofter's wife
Alexander Munro, grandson, 11, scholar, b. Forgue
There is a birth registered in 1880 at Ythan Wells for an Alexander Munro, which is also indexed as Alexander Alexander, so both (unmarried) parents must have signed the register.
The quoad sacra parish of Ythan Wells was formed from part of Forgue parish and part of Auchterless parish.