Moisture is trapped under some part of the surface finish. You need to get it out. I try alcohol first. Alcohol dehydrates. Anyone who’s had one-too-many cocktails knows this. Pass a damp, alcohol-infused cotton swab or cloth over the ring, not on it, hover just above it. If it works, the moisture will be drawn from the wood and the ring will disappear without your hand ever touching the surface. I’ve done this, and to my absolute astonishment it worked…like furniture repair magic. Trust me on this, your sense of accomplishment and amazement will be off the charts. You’ll want to show all your friends.
If that doesn't work, my go-to alternative is Restor-A-Finish, about $10 at Lowes or Home Depot. Follow the directions carefully.
If that doesn't work, go to YouTube and type in white ring repair. You can spend the entire day watching how to get rid of your ring videos; then spend the next week trying them all! Enjoy the journey.