Looks like old dried epoxy or some other form of glue. Or maybe it started off as tree sap; if you get lucky you might even find a mosquito frozen in amber! Perhaps he was the most hated guy in the club and it's an old loogie (lugie?) that never got wiped off.
I try not to think too much about what precisely I'm cleaning off of a new acquisition.
Oh, and those are clearly alien spores inside the seat tube. Clean or quarantine, but do it quick.
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A race bike in any era is a highly personal choice that at its "best" balances the requirements of fit, weight, handling, durability and cost tempered by the willingness to toss it and oneself down the pavement at considerable speed. ~Bandera