I've had a bottle terrarium since May last year, and it has seemed quite healthy for most of that time. The plants have at least grown very well. It has a substantial layer of sand and gravel topped with charcoal to hold water, and it has neither appeared overly wet nor too dry, so I have not added water since we constructed it in May.
In the last week a layer of green stuff, algae I assume, has spread around the top of the glass. There are a few places with big spots up to 1.5mm across, while there's a layer of what looks like mixed condensation and algae covering the rest of the glass. There's also some thicker algae directly above the soil (not really visible in the below photo).
A Reddit discussion suggests that sunlight leads to algae growth, which would make sense for ours as we recently moved it because of Christmas decorations into a location which would get a small amount of direct sunlight in the mornings. This page says that it can just be cleaned with a paper towel and maybe a little vinegar.
Does sunlight sound like a likely cause, and if so, after cleaning the glass well, will moving the terrarium to a location which won't get sunlight prevent it from growing again?