I'm assuming that you mean the white power on the black rubber?
If the lens is new, it's typically a form of mold release so that the rubber in the die doesn't get stuck when it's cooled and ejected from the mold. Automated molding processes are supposed to eject a precise amount whereas human interacted molding is, well, human. Sometimes, they just spray too much. For the automated processes, sometimes the hopper gets low and the operator will dump in a fresh load causing some air in the line to eject a glob of powder.
If it's older, it's usually the rubber reacting to atmospheric components. "Drying out" as you may. The rubber contains some oils and as those outgas, the reaction provides the white residue. Car tires do it, just not as white.
Either way, it's harmless. If you're paranoid, just wipe it off with a damp towel and add some "Tire Shine" to it. In the summer, I'll actually add some billard chalk to my 1Ds3's grip rubber so that it doesn't get slippery with sweat...you could always use rosin too...then again, a lot of "tire-g" and you'll be bling'ing.
