Recently I was having some problems with dirt in my carburetor. And, I decided to flush the gas tank and clean the sediment bulb in my 1926 TuDor. So I got out one of my larger gas cans and removed the fuel from the tank.
I removed the sediment bulb and cleaned out a very small amount of white dusty stuff and a little brown dirt. I put a number of gallons of fuel through the tank to flush it out. The sediment bulb was reassembled and lubed with EZTurn and put back on the car. Then it struck me to look in the gas can to see how clean it was. It was not.
There was dirt, bugs, rust, etc. in there. So I decided to flush out the gas can into another can and found that that can was just as dirty.
Spent yesterday afternoon cleaning all my gas cans. I'm sure if you are like me you have a bunch of them and they lay around on the floor or in a corner. Some of them have covers some are open. So my lesson here is to watch the insides of the cans and try to keep them closed (also a safety item) and clean.
IIRC this was on the fuel caps on Fordson tractors: "Buy clean fuel, keep it clean"
The white dust you describe is the result of the alcohol in modern fuels dissolving the "varnish" left by older fuel. Old type fuel won't dissolve the varnish it leaves behind by evaporating but the alcohol in modern fuels will and results in the white dust you're finding. Insanity resulted at the marinas here when the switch was made years ago which is how I found out about it.
BEWARE of older boats with built in glass tanks. The ethanol attacks the resin and turns it to goo, getting as far as the top end of the motor fouling rings and valves. Its become a real issue on older Hatteras 'glass tanks especially.
Ethanol will also dissolve the "OLD TYPE TANK SEALANT". That mess will plug up everything. Dan