looks like corrosion, but for the very same reason Bogie stated I DON'T think it occured in the carb.
It affected the NEEDLE AND SEAT. which to me says it came from the line and into the carb. Plus the carb is too new to corrode like that- if it was sitting around for a LONG time then yes I would suspect the carb but 3 weeks cant do that to a new carb.
I had a very similar problem a while back, to remedy it I ran the tank near empty with a 50/50 mix of gas and E85 (only a couple gallons of the mix at a time). While it is running through the engine I had to clean the carbs multiple times but its sure as hell a lot easier than cleaning the tank and lines themselves. The idea is to flush them clean.
My approach takes time but it worked. E85 was recommended to me through this forum BTW, one of the many good useful things it has taught me