The anode does not have a switch. I think you are referring to the electric heating element. So you were running both gas and electric heating at the same time, and the manufacturer and the user's manual say it is OK to do this.
As you thought, the yellow color probably means there is something going on in the water heater. It could be rust, it could be some sort of algae or other growth, or it could be minerals. If it is rust - the color should be redder than yellow - the unit is on its last legs. The tank is probably glass-lined so it won't rust. But I think this is unlikely. If it is some kind of scum, you can probably clean it out by heating and then draining several tanks full of water, perhaps with something like bleach in the water to kill the beasties. Be sure you turn off the electric heating element before draining - it will burn out if you don't. You need to look for advice on how to do this. If it is mineral deposits, something like vinegar may dissolve them. And again, you need to look for advice.
How long was the tank sitting around unused but full of water?
Bill
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