What to do with filler neck

Whow never seen that! I soaked mine in evaporust for about 4 hours and it came out clean and brite!

Not sure what to make of that! Was the solution overused?

I have noticed that some parts only need a few hours soak, exspecialy galanized type pieces. IE window regulators.
It was a brand new bottle. I checked the filler neck regularly while it was soaking. This happened and hasn't gone away since.

I am considering soaking part of it in vinegar to see if it's able to remove the yellow crusting. Thoughts/opinions?