On a much larger image it looks like a foam structure containing eggs, or that's just the foam structure. In the UK there is an insect commonly called a " frog hopper' that creates this foam structure around itself. It's a little green insect that tends to live where the branch meets the main stem, on smaller plants. It's probably got it's common name as it's structure resembles the foam nest of the frog that produces the foam when egg laying.It's definitely no mould. It appeared yesterday and disappeared without traces over night.
The enclosure is an ExoTerra, ventilation should be enough.