When I worked in the church nursery, some of the 1- and 2- year olds called me "mom". And not just me, but the same children would call the other nursery workers "mom" too. In their little minds, "mom" must have equalled "the person in charge who would give you snacks". :) As they got bigger and more adept at speaking and thinking, they started calling me by my own name.
It's nice that this little girl has your husband to give her a picture of what a daddy is like. She hears your daughter calling him "daddy" (and if you're like our family, you often call your husband "daddy" also) so to her "daddy" is his name. If you would like to make the distinction that his name is "Mike" (or whatever), then you'll want to consistently use the name Mike around her. When she makes a statement about "daddy", you can repeat it in affirmation, but substitute "Mike", like, "Yes, Mike will be right back with your crackers."
It will help her to see that only your daughter calls him daddy, and that to other people his name is Mike.