This is William Dawson. Yes, the first letter looks like an 'M', but it is actually William - look very carefully! He appears in the 1841 Census returns in Dingwall, aged 60 [i.e., within the age range 60-64 - ages of those aged over 15 were rounded down in the 1841 Census returns], this time, however, with a Catherine, aged 50 [i.e., 50-54], but with "Betsy" (Elizabeth Dawson, aged 10) with them in the household.
Did he remarry between 1841 and 1851, or could Catherine have been a Catherine Ann and she was recorded wrongly simply as Ann in 1851? You'll have to research that ...