To add on @ppumkin's post
The USB cable has four wires, whereas two of those are for power (VCC and GND). The other two are data lines and not connected if the power is coming from a wall wart .
You can connect the PI and the pump to the same power supply if
- The pump actually runs from +5V
- The combined current draw from PI+pump is below/equal the maximum current rating of your wall wart
Then you can simply connect two wires to each the +5V and the GND with one 5V/GND pair going to the PI and the other pair going to the pump.
A quick google search showed me that there are indeed small 5V pumps. The start current on the pump was 2A. Combined with the PI the wall wart should provide ~3A.
Do not connect the other end of the micro-USB cable to your computer. Computer USB ports are rated at 500mA max, which would be 6x too low if we assume a 3A current requirement. This could potentially damage your computer.