I have never seen this either. I have seen plenty of carbs that are gummed up or full of dried whitish stuff. I have been looking into what to do and some have posted on other tractor forums that E15 can dissolve this stuff created from E10. I am not sure if they are reffering to the same substance I have. It is hard to tell. I do not know if this is some wives tale or if their is any truth to it.
Where I live only E10 is available at gas stations. I have never in a decade or more seen E0 at the pump. I have never seen E15. So my conclusion is, it must be E10. Unless the gas was transported from hundreds of miles away where another type of gas may be sold. This is a mystery. I have never seen anything exactly like this.
That "whitish stuff" is mostly cast aluminum deteriorating from Ethanol fuel exposure. Ethanol fuel is hygroscopic because it draws in moisture from the air. Aluminum is a self-sacrificial metal that forms a white coating when exposed to moisture unless it has a high Nickel content. Carburetors have very little Nickel blended with the aluminum when cast.
Some of this stuff is also specialty additives that are blended in the fuel during the refining process.
The stuff you are showing was not created by E10 or any other fuel, it was carried by it when pumped from a bad ground storage tank with sealer that has been added as I have explained already. It could have also came from a contaminated tanker load that was off loaded at an unsuspecting station.
You would have to actually locate the fuel tanker depot supplier or a refinery level technician before you might get lucky enough to find someone who knows what this stuff is. Most will play ignorant until it gets bad enough to end up in court.
Ground tank contamination is real but kept well hidden. Rural areas usually suffer the most.