Can current also flow from low potential to high potential? 
I understand that current flows from high potential to low potential.
As you can see in the picture, the top of the circuit has a potential 2V higher than the bottom.
However, since the current is shown as -2A, it actually flows from the low potential side (bottom, -) to the high potential side (top, +).
Is this possible case?
(※The Square is simply a 'Device'. In this case, the Device supplies energy (-4W).)
 A: Current can flow from low potential to high potential. In fact, in any circuit where potential isn't the same everywhere and there is current flowing, current must flow from low potential to high potential in some part of the circuit. Note that when you hook up a resistor to a battery, current is flowing from high potential to low potential in the resistor, but from low potential to high potential in the battery.
At steady state (when potentials and currents are not changing in time), this can happen in devices that are sources of some type of electromotive force (emf), such as a battery, solar cell, thermocouple (through the Seebeck effect) or a piece of circuit moving in a magnetic field (motional emf).
A: By convention, the electric current positive direction is the direction in which positive charge moves. It is unclear in your schematic what the "rectangle on the right" represents: a voltage source, a resistor, or something else?
I would read the current in your schematic as follows:

*

*current magnitude is $2 \text{ A}$

*positive charge moves from right to left; hence, potential on the right is higher than potential on the left

If you are still confused by the negative sign, just flip the arrow of the current direction (rotate by 180$^\circ$) and put positive sign next to the current.

Can current also flow from low potential to high potential?

In the absence of external forces, positive charge always moves from higher to lower potential, much like objects fall from higher to lower altitude. However, charges can go from lower to higher potential if there was something providing the force necessary to overcome the potential difference; e.g. a voltage source (battery). This is the same as you climbing to a higher altitude by using an elevator.
It is also possible for positive charge to move from lower potential to higher potential by means of power converters. These use semiconductors (diodes and transistors), inductors and capacitors to transfer energy (power) from input to output. Physics still apply to power converters, but it is the way how they work that enables positive charge to flow from lower to higher potential.
