Green ring in bottle neck?

A year or so ago I made an American Lager from a kit. I did everything the instructions said to do and everything looked and smelled normal during bottling. I placed the primed and capped bottles in my basement, on shelves that I covered with an old table cloth to prevent any light from hitting the bottles. The temperature remained close to 60F. I let the beer age a month or so before pulling out a bottle to try. I noticed the bottle had a green ring around the neck of the bottle where the beer level was. I checked the rest of the batch and all of them looked the same. Upon opening the bottle foam immediately began pouring out of the bottle and I noticed a slightly strange odor. Not a bad odor, but not like most beers I've smelled. I began opening and dumping the bottles out because I assumed they were bad. However, I couldn't stand to watch beer go down the sink, so I tried some. It didn't taste too bad. I did not have the courage to drink more than a few sips.
Any ideas what happened to it?

The ring around the bottle neck is a sure-fire sign of bacterial contamination. What were your methods of sanitation?

If the beer smells and tastes fine, it could be a film from priming sugar that wasn't mixed well.

Don't be so quick to pour your batched brews out -- more often than not it can be salvaged and enjoyed!

It sounds like some kind of bug for sure. If you like the taste of it, you may as well enjoy them.