If this happened to me on national TV, I'm almost certain it'd signal the crash-and-burn end of my political career:
I'm sure that Mr. Cruz himself will say that the bit of schmutz on his lip was just a tiny, runaway ort of food, maybe corn or some such—pay it no mind. People on Twitter, excitable as they are, are calling it a "booger." I suspect, given its size, shape, and color, that it was a tonsillolith, which would have made for some highly unpleasant eating.
NB: I don't know when the above footage was taken, so I really can't say whether this was, in truth, a "Dean Scream" moment. If it happened years ago, the resurfacing of this image reeks of suspicious timing. If it happened during a recent GOP debate... d'oh.
ADDENDUM: fellow Twitter denizen Barry White confirms the "booger" is from last night's GOP debate. Barry even provides an interesting link.