First let's define a few terms. An element is a pure chemical in its simplest form that contains a single type of atom. An example is carbon. Pure carbon will be composed entirely of carbon atoms with no other atoms mixed it. A chemical compound is a single substance that is composed or more than one element. Water is an example. A sample of water is a single substance where every molecule is H2O (one atom of oxygen and two atoms of hydrogen combined). A mixture is a medium that is composed of more than one substance. The mixture can be either entirely uniform (homogeneous) or non-uniform (heterogeneous).
In your example, the clear liquid eventually ends up as a solid over time. Since you can see that two different substances are involved with different phases of matter (a solid and a liquid), we know that the original clear liquid is a mixture. It is most likely a solid like salt or sugar dissolved in water. That contains two different substances combined, hence it is a mixture.