Its probably grease, which keeps the rubber seals alive. If you want your top to remain leak free, make sure you grease the crap out of the rubber every 6 months or so with a quality lithium grease. The best IMHO is the Honda shinetsu grease, which works magic on rubber.
Honda Genuine Shin-Etsu Grease : Amazon.com : Automotive
My top is almost 2 years old and never so much as a drop of water. Without grease, your rubber will dry out (aka shrink) and start letting water in. A tube of this stuff should last you 4 years if you grease every 6 months.