White mould on the bark: when I opened the jar and when I waited
Both jars were set up on the same afternoon with the same substrate mix, the same batch of cushion moss and a piece of cork bark each, cut from the same larger slab. Four days later both had a soft white fuzz on the bark, mostly on the underside where it touched the soil, with a few threads reaching up onto the moss. This is common with any wood that has not been boiled, and I knew that, but it is still alarming to watch.
I treated the two jars differently on purpose. Jar A was opened, the bark lifted out, the fuzz wiped off with a cotton bud dipped in a weak hydrogen peroxide solution, the 3 per cent kind diluted again by half, and the lid left off for six hours to dry the surface before resealing. Jar B was left closed and ignored, apart from being moved 20 centimetres further from the window so it ran a little cooler.
Jar A: the mould came back within three days in the same place, slightly less of it. I repeated the treatment. It came back again, and this time a patch of the moss nearest the bark had gone pale and limp, which I think was the peroxide rather than the mould. After the third round I gave up intervening.
Jar B: the fuzz spread for about a week until it covered most of the bark underside, then stopped. Over the following fortnight it thinned, greyed and collapsed into a faint film. By day 25 it was gone and the bark had a dark, slightly glossy look. I found springtails in there a week later, tiny pale things jumping on the condensation, which I had not introduced and which presumably came with the moss. They are still there, and I have not seen mould in that jar since.
The lesson I have taken is that an early mould bloom in a fresh jar is usually a transition rather than a disaster, and that opening the lid resets the humidity and sets the whole thing back to day one. I now leave a new jar shut for a month before I let myself judge it, and I pre-soak any bark in boiled water for an hour before it goes in. Jar A recovered eventually as well, but it took until the summer.