What is this and how do I get rid of it? I have no idea what that white ring is or how I applied it. Here's a picture of it in edit mode: 
I want to get rid of it but I don't know how to go about doing so. I'd appreciate some help. If needed, here is the file: https://pasteall.org/blend/07f0ab12a84944e69b2afd50cf7d3f6f
The issue is being caused by the presence of an extra face inside the mug. Its a quick fix; in edit mode, select it and press  X  > Faces.



Also note that there's an Ngon near the base of the mug that could use some splitting, for uniformity's sake. In edit mode, select the top and bottom vertices and press  J .



This should do it, good luck!