Most likely it is light debris, stuff like leaves, dirt, anything that literally is not bolted hard to the ground. They clear the pad of obvious loose stuff, almost like on an aircraft carrier where they sweep for FOD (Foreign object debris) but there is a lot of ground around a launch pad.
Also, recall the engines of a rocket, unlike a plane on a carrier or even a normal runway, do not inject air rather it exhausts huge amounts, so almost anything that does blow, will blow away (far away) instead of being sucked into an engine. (Eagles may soar, but weasles don't get sucked into jet engines).
If you look a bit earlier you will see a splash of dark stuff come up the left side of the rockets as well. That is water from the Niagara deluge system, which is designed to protect the pad from acoustic damage. The system dumps amazingly large amounts of water, which the heat boils, instead of the concrete, and the water absorbs the sound (also causing it to heat up) instead of vibrating the concrete.
Other launches have had the dirty looking water splash pretty high up and get the pretty booster dirty. But as we saw in Orbcomm OG2 launch and landing, coming back, through your own RP1/LOX exhaust is pretty dirty work from all the soot on the first stage.
Thus the water minimizes the physical damage to a lot of the pad surface. However, a 1.3 million lb thrust rocket just lit on top of the pad and flew away, stuff is going to be damaged. The trick is to make the expensive stuff resilient and the stuff that is going to get destroyed anyway, cheap and easily replaceable.
SpaceX is clearly far along in the learning phase as they just completed their 21st launch campaign, and they seem to be getting better at resetting for the next launch.
