Resin frequently crystallizes or changes its consistency to a milky substance. This commonly happens when exposed to temperatures below 50 degrees, when stored on concrete, or when placed on metal surfaces. I am aware that it is contained in a plastic bag. But the chemicals are affected by being near the ground or a metal surface.

What should I do if the epoxy resin starts to crystallize or become milky?

Usually, the altering of the resin begins at the bottom of the container.

How can this be fixed? 

A quick repair is to submerge the bottle in warm water for around 15 minutes (make sure no water gets inside).

Place the resin on top of wood or double it up on cardboard surfaces for future storage.