Look like there was a reaction between the evaporust and the zinc based coating on the filler neck. The filler necks have a galvanized kind of finish that's zinc based. I would guess that because the tube was in there for 3 days the evaporust reacted with the zinc to cause the yellow color. Not sure it would be a problem, just like the white-ish appearance isn't a problem on the filler neck.
Also, I don't think you're going to totally get rid of that stuff unless you blast it, or etch it in an acid bath. And if you do either of those things, you'll need to have it re-galvanized if you don't want it to rust.
