There are no Slark's or Slack's on the Isle of Man in 1841, and just the one couple in 1851:

SLACK Henry 59 Malew 2526 8 30
SLACK Jane 64 Malew 2526 8 30

Given their ages, I suppose it is possible that they had children who came to the Isle of Man with them sometime between 1841 and 1851, who may have subsequently emigrated.