I am going to presume that by "square prism" you mean "cube." The difference is that a cube has 6 identical square faces, where as a square prism is actually a rectangular prism that need only have two opposing faces (or sides) that are square. I make the presumption of a cube because you only provided a single dimension and the problem is not solvable with only a single dimension using the more general definition of a square prism.
The total surface area of a prism is the sum of the areas of the sides. A cube has six identical sides and the area of each is the square of the measure of an edge. I don't know if you mean that each side is 6 square units or you mistakenly used "side" to mean "edge" and that each edge measures 6 linear units, so you will have to take it from this point.
John
My calculator said it, I believe it, that settles it
