Why do slime molds pulsate when growing into their fruiting body?
What you have observed is called shuttle streaming. This is how Physarum polycephalum (I assume this is the species of slime mold in the video) and many other slime molds move.
I found this here;

The movement of P. polycephalum is termed shuttle streaming. Shuttle streaming is characterized by the rhythmic back-and-forth flow of the protoplasm; the time interval is approximately two minutes. The forces of the streaming vary for each type of microplasmodium.
The force in amoeboid microplasmodia is generated by contraction and relaxation of a membranous layer probably consisting of actin (type of filament associated with contraction). The filament layer creates a pressure gradient, over which the protoplasm flows within limits of the cell periphery.
The force behind streaming in the dumbbell-shaped microplasmodia is generated by volume changes in both the periphery of the cell and in the invagination system of the cell membrane.