Edit: It can almost certainly be excluded that the white spot was caused by Curiosity's ChemCam's laser, because it doesn't make sense that the laser was used at that time without taking images with ChemCam.
The only images captured by ChemCam on sol 550 were taken at least 1 hour before and at least 3 hours after the white stuff appeared on the surface of Mars, so it's highly unlikely that when that happened the laser was used.
The image above was captured by the Mars Hand Lens Imager (MAHLI) on sol 550 (february 22, 2014).
4 minutes later, white stuff seems to have emerged from one of the holes on that image, shown in the image below.
The pictures above are part of a sequence of raw images shown on the mars.nasa.gov site. 6 images captured in 30 sec. show this white stuff, after that another feature was imaged.
What could this be ?