Skimmer Cup Striations

[Parker]

So I'm cleaning out my skimmer cup and I find these inside the neck of my skimmer cup. Anyone know what might cause this? There is also some sort of film inside of the cup, this was taken after soaking in a vinegar bath for hours. The cup isn't exposed to heat of any kind, they are on the inner neck so there isn't any weight on the cup.. Very odd. The surface is now a tad bit rough, so I'm a little worried now about it's effectiveness.