Welcome to NGA,
@BSlatt .
Many years ago I bought a "bottle terrarium" kit, with a large -- 3-gallon, maybe? -- jug with a narrow neck. It took me a couple of tries but I was eventually able to get it planted and "balanced" (as far as moisture) and it thrived for quite a long time. At some point I decided to tear it all apart, because things were overgrown and no longer attractive, and then never "got around" to replanting it; I still have the jug and now that I'm thinking about it I might add that to this winter's to-do list.
You had the right idea about the mold -- if mold is forming, there is too much moisture and you need to take the cap off. Watering needs to be done very sparingly; I had my bottle planted horizontally and used a small spoon fastened to the end of a dowel stick to deliver small amounts of water to each plant. For a vertically planted jug, such as you have, I would try using a drinking straw, or maybe a piece of tubing like that used for aquariums, to direct the water to the individual plants. A fork fastened to the same type of stick is helpful for planting or adjusting your plants, as is a "grabber" thing like this:
https://www.amazon.com/dp/B000RB7AGY/
It definitely takes a good bit of patience to deal with a terrarium planted in a jug, but it's fun to have once you've done so! If you don't want the hassle involved with trying to keep live plants healthy in it, you could always look for some realistic-looking artificial plants and use those!
