[RunsWthSzissors]

The clear fluid is serous fluid, part of the the interstitial fluid which helps make up the the extra cellular matrix present to some degree in all tissues. The vacuum of the cylinders can literally pull it out of tissue through the skin if the outer cornified layers of skin cells are thin enough, or in the case of your nipples, from the epithelial tissue lining the ducts and glands just under the nipples. It's harmless and other that the negative effects of blisters (which also fill with serous fluid) you don't need to worry about it.