Mixtures by definition are variable in composition (e.g. more than one substance), otherwise, they would be called a pure substance of a compound or element.
A mixture uniform in appearance is called a solution.
The mixture is a supersaturated solution.
It is possible to create a supersaturated solution by heating the solvent and adding more solute than could be dissolved at room temperature. When the supersaturated solution has cooled to room temperature, if it is disturbed, such as bumping it, or if it is seeded with a tiny amount of the solute, the excess solute will come out of solution and crystallize.
The video shows what happens to a supersaturated solution of sodium acetate when it is seeded with a tiny amount of the solute sodium acetate.