What is white stuff in my soil/box?

Perlite is a commercially used mineral that is added to soil mixes to help with soil drainage. Generally used more often in cactus soil mix but we also use with succulents.  It is not broken packing popcorn so don't spend time trying to pick it out! 

See Blog picture below for example.

http://thesucculentsource.com/blogs/succulent-cactus-news/100238209-my-succulents-fell-out-of-their-containers