Carb vent line build up

I've got a '90 KDX200. Small amount of gas leaking from vent tubes, need to adjust the float. But, I'm getting a white substance building up at the end of the tubes. It looks like and is about the size of a piece of popcorn (smell like gas of course). When I pick it off, after a couple of seconds it sort of melts or dienegrates. I cleaned the carb before my last ride.

Any ideas on what this is?

Thanks, ADCKDX