Hi
Further to the above from Looby the 1851 census has
John Johnston unm. 26 Farmer 25 acres b Elgin
Margaret Johnston wid 47 b Elgin
Alexander son 5 b Moray
Address Rashcrook, Birnie (same as 1841 census).
Then there is this death notice in the Elgin Courant and Morayshire Advertiser 18 July 1851. " At Rashcrook, parish of Birnie, on the 8th inst., Mr John Johnston, aged 28 - much regretted".
The census entry would seem to confirm the implication from the wording of the birth for Alexander that he was illegitimate.
Have you tried looking at the libindx for John and Diana?
William