The shot exhibits what is called ghosting, and it occurs when you shoot with
flash at extremely slow shutter speeds and there is subject movement.

The flash freezes the subject normally, but at this slow speed, there is enough
ambient light in the exposure to illuminate the movement of the subject.

That's why [there is] not only the blurring of the subject but the weird color which is a mix
of the room light and the flash. When you shoot an event like this in low light, put the
camera in manual, the shutter speed on 1/125, and adjust the aperture for the desired DOF.

Keep the ISO at or below 800. Experiment with some shots and see how you like the look. If
under or over exposed, adjust the FEC on the flash accordingly. If you want to bring in more
ambient light, you can slow the shutter speed but when you go below 1/60, you begin to run the
risk of the ghosting problem.

The red lines in the shot are from the [contrast] focus assist light on the flash, which throws
out a hatched pattern in red and black to help the camera focus in low light. Comments by Bruce Ward